Add persistent sketch-constraint system (ocs_gcs port) and named-parameters handoff docs

Ports FreeCAD's planegcs 2D geometric constraint solver to Rust as a new
crates/ocs_gcs workspace member, and wires it into the document/scene model
as a persistent SketchConstraintSet: Coincident, Horizontal/Vertical,
Parallel/Perpendicular, Equal, Tangent, Distance, Angle and Radius
constraints that re-solve live (including mid-drag, not just on release),
render glyphs and a DOF badge, flag conflicts with a one-click resolver
pill, and round-trip through save/load via an XRecord blob.

Also includes docs/command_reference.md (a statically generated reference
of every registered command and its prompt sequence) and
docs/named_parameters_design.md + docs/backlog.md, which hand off the next
queued feature (a named-parameter/expression table) and consolidate every
deliberate scope-down and open gap found along the way, so a fresh session
has a complete starting point.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

docs/backlog.md

@@ -0,0 +1,148 @@
+# Backlog — deferred items, known gaps, and scope-downs
+
+A consolidated list of things noticed but deliberately not fixed/built, so
+they aren't lost between sessions. Pulled from `docs/
+parametric_system_design.md`'s own "open questions" log, the `ocs_gcs` port
+plan, and things found while doing unrelated work this session. Not a bug
+tracker for the whole app — scoped to what surfaced during the parametric
+constraint system project and its surrounding investigation.
+
+## Parametric constraint system — deliberate scope-downs (documented, not accidental)
+
+1. **Stage 9 undo is two presses, not one.** Adding a persistent constraint
+   and the geometry move it triggers are two separate undo/redo entries
+   (`HistorySnapshot::SketchConstraints` pushed adjacent to, not merged into,
+   the geometry `Delta`). The design doc's recommended §5.1(b) "live
+   XRecord" approach would fix this but depends on stage 4 (save/load),
+   which itself was scoped down (next item) — revisit both together if this
+   is ever worth fixing.
+2. **Stage 4 does not resolve on load.** Opening a file with saved
+   constraints restores them but does not immediately re-solve — the DOF
+   badge and any conflict state only appear after the next edit that touches
+   an affected scope. Deliberate: resolving on load risked desyncing from
+   the `DerivedCaches` bundle `on_file_opened` installs right after, without
+   budget this session to verify that's safe.
+3. **Stage 11's conflict resolver is a one-at-a-time pill, not the full
+   SketchXpert-style panel.** Clicking "⚠ n conflicting" removes one flagged
+   constraint and re-solves; there's no named-candidate list or cyclable live
+   preview before committing (design doc §6.4 Phase C). This is the single
+   largest UX gap remaining in the constraint system relative to the
+   original plan.
+
+## `ocs_gcs` solver — not ported
+
+- `SlopeAtBSplineKnot` (tangent continuity across a spline knot) — needs
+  knot-multiplicity reconstruction for a narrower use case than
+  `PointOnBSpline`, which is ported. Deferred, not silently dropped.
+- `InternalAlignmentPoint2Ellipse` / `InternalAlignmentPoint2Hyperbola` —
+  FreeCAD-Sketcher-internal bookkeeping for *creating* conic geometry
+  interactively, not for constraining existing geometry. Lower priority.
+- `EllipticalArcRangeToEndPoints` — dead code upstream (a `ConstraintType`
+  enum value with no implementing class anywhere in current planegcs
+  source), not a gap in this port.
+- **Golden-fixture testing against the locally-installed FreeCAD.app**
+  (testing-strategy "Oracle A") was never wired up. Everything in `ocs_gcs`
+  is verified only against Oracle B (closed-form + finite-difference). Still
+  a real asset sitting unused if higher-confidence validation is ever
+  wanted.
+- **Performance at scale is untested.** The "only touched scopes rebuild"
+  locality argument (design doc §4.2) is architecturally sound but has no
+  profiling behind it on a large, heavily-constrained drawing.
+
+## Constraint types not exposed in the app's UI (even though some primitives exist)
+
+`ConstraintKind` (`src/scene/sketch_constraints.rs`) currently has:
+Coincident, Horizontal, Vertical, Parallel, Perpendicular, Equal, Distance,
+Angle, Radius, Tangent. AutoCAD's own Geometric Constraints panel additionally
+has **Fix**, **Concentric**, **Collinear**, **Symmetric**, **Smooth** — none
+of these have a `ConstraintKind` variant here. Some are buildable from
+already-ported `ocs_gcs` primitives (e.g. Concentric ≈ `C2CDistance` at zero
+distance) without new solver math; `Fix` would need a different mechanism
+entirely (pinning a point's params non-free rather than adding a constraint
+row) and doesn't fit the current `ConstraintKind` model at all.
+
+## Newly discovered this session (not previously tracked anywhere)
+
+- **`CCONSTRAINT`, `TCONSTRAINT`, and their four siblings
+  (`HCONSTRAINT`/`VCONSTRAINT`/`PCONSTRAINT`/`QCONSTRAINT`/`ECONSTRAINT`) have
+  no `inventory::submit!(CommandRegistration)` registration.** Found while
+  generating `docs/command_reference.md`. All seven work correctly when
+  invoked (ribbon button or exact typed name) but don't appear in the
+  command-line's autocomplete or in the MCP server's `commands` listing,
+  unlike almost every other command in the app. Pre-existing across the
+  whole constraint-command family, not something this session's Tangent/
+  Coincident additions broke uniquely — just never noticed before. Cheap fix
+  (one `inventory::submit!` block) whenever someone wants to spend five
+  minutes on it.
+- **No live link from a sketch to 3D solid features.**
+  `SolidHistorySweep`/`Revolve`/etc. (`src/scene/model/solid_history.rs`)
+  hold `sweep_entity: Option<EmbeddedEntity>` — a geometry copy snapshotted
+  at creation time, not a `Handle` back into the sketch. Dragging or
+  re-solving a sketch that was previously extruded/revolved/swept does not
+  update the resulting solid. This is the real "feature-history tree" gap
+  discussed when scoping FreeCAD/AutoCAD parity — a substantially bigger
+  project than the named-parameters work (see `docs/
+  named_parameters_design.md` §3), not something to fold into it by
+  accident.
+- **No named-parameter/expression sheet** (AutoCAD's Parameters Manager /
+  FreeCAD's Spreadsheet equivalent). This is the next feature queued up —
+  see `docs/named_parameters_design.md` for the handoff context.
+- **No end-user manual exists anywhere in the repo.** Checked `docs/`
+  thoroughly — everything there is developer-facing (plugin architecture,
+  this parametric design doc, tessellation notes, native-vs-web diffing,
+  release notes, automation scripts) plus localized top-level README
+  translations. No in-app Help link points anywhere either. Not started;
+  would be a real, separate writing effort if wanted.
+- **No theme install/import/export mechanism.** Theming is entirely in-app:
+  pick one of iced's built-in themes, or hand-tune one "Custom" slot via six
+  hex-color inputs (`src/ui/window/options.rs`, `UiThemePalette` in
+  `src/app/config.rs`). No theme file format, no way to author and share a
+  theme, only one custom palette remembered at a time.
+- **No touch/multi-touch input support**, relevant to the wasm build running
+  on a tablet (e.g. iPad). Grepped the whole codebase for touch/gesture
+  handling — none exists; all input assumes mouse+keyboard, relying on the
+  browser's synthetic mouse-event emulation for taps. No pinch-zoom, no
+  two-finger pan, and OSNAP/grip-drag precision (already tight even with a
+  mouse — see the Coincident epsilon note below) would be materially worse
+  with a fingertip.
+
+## Testing/tooling notes for future live-verification sessions
+
+- **Coincident's success path could not be reliably live-verified via
+  synthetic mouse clicks** — its resolver epsilon (`COINCIDENT_EPSILON_SQ =
+  1.0e-12`, ~1e-6 world units, reused from stage 8's `infer_coincident_refs`)
+  is tighter than this session's `left_click`/OSNAP automation could
+  reliably hit, even with Endpoint snap enabled. Concluded to be an
+  environment limitation, not a code defect — automated tests cover the
+  resolver logic directly — but worth knowing before assuming a future
+  automated click-through will succeed where this session's didn't.
+- **`open_application`/`request_access` always resolve to the
+  `/Applications`-installed app bundle**, regardless of which binary was
+  most recently built. To live-test a code change, build a release binary
+  and swap it into a **scratch copy** of the installed bundle (e.g.
+  `/tmp/ocs_livetest/OpenCADStudio.app`), keeping the original small
+  launcher (`CFBundleExecutable = "OpenCADStudio"`) intact, then
+  `codesign --force --deep -s -` to re-sign ad-hoc before `open`-ing it.
+  This reuses the real bundle identity (`io.github.HakanSeven12.OpenCadStudio`)
+  so access requests correctly target it. A standalone bundle with a fresh
+  bundle id gets silently `user_denied` — not a viable shortcut. The debug
+  binary run bare via `nohup` never presented a controllable window either
+  — use a release build.
+- **`target/debug` can balloon fast from repeated live-testing rebuilds** —
+  hit 35GB and caused a real "No space left on device" build failure this
+  session. `cargo clean` reclaimed 64GB. Worth a proactive `cargo clean` (or
+  checking `df`) before a long live-testing stretch, not just after it
+  breaks.
+- Synthetic input reliability, generally: Escape frequently failed to cancel
+  an active command (use Return instead); LINE clicks sometimes produced
+  stray zero-length duplicate entities; selection clicks often missed thin
+  line bodies or landed on a midpoint-edit-box zone (use a crossing-window
+  drag-select instead of precise body clicks).
+
+## Legal / process
+
+- **LGPL-2.1-or-later review for `ocs_gcs` is still open** (original plan
+  §6). Non-blocking for continued development, but must happen before any
+  public release containing it — separate-relinkable-module posture vs.
+  broader relicensing, notice/attribution obligations under GPLv3
+  distribution.