refactor(scene): group modules into role-based subfolders

Move the 30 flat modules under src/scene/ into six folders by role:
convert (entity→geometry), text (fonts/shaping), model (render models),
pick (hit-test/selection/grips/xclip), view (camera/transform/render),
cache (block + property caches). pipeline/ is unchanged.

Full path migration — all `scene::X` references become
`scene::<group>::X`, no compatibility re-exports. render.rs items that
were `pub(super)` (parent was scene) become `pub(in crate::scene)` to
keep their original scene-wide visibility.

No behavior change; build and tests are green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/scene/convert/tess_util.rs

@@ -0,0 +1,98 @@
+// Shared helpers used by per-entity tessellation impls in `crate::entities`
+// and the dispatcher in `crate::scene::convert::tessellate`.
+//
+// Anything cross-entity (colour resolution, arc sampling, chord tolerance)
+// lives here. Entity-specific helpers (dim format strings, mleader path
+// types, etc.) stay with their entity file.
+
+use acadrust::types::Color as AcadColor;
+use glam::Vec3;
+
+use crate::scene::model::wire_model::{SnapHint, TangentGeom, WireModel};
+
+/// Output of the fallback per-entity geometry path used by entities not
+/// covered by the truck topology pipeline (Viewport, Insert, Hatch
+/// outline, Ole2Frame). Tuple form preserved to avoid touching every
+/// callsite when the dispatcher wraps these into a WireModel.
+///
+/// Layout: `(points, snap_pts, tangent_geoms, key_vertices)`.
+pub type FallbackGeometry = (
+    Vec<[f32; 3]>,
+    Vec<(Vec3, SnapHint)>,
+    Vec<TangentGeom>,
+    Vec<[f32; 3]>,
+);
+
+// ── Arc tessellation helpers ─────────────────────────────────────────────
+
+/// Convert hatch-boundary arc `(start, end, ccw)` into a
+/// `(start, signed_span)` ready for the sampling loop. Matches the
+/// legacy `(TAU - sa, TAU - ea)` flip used here for years — direction
+/// semantics are preserved on real files. (Wrap-through-2π is a known
+/// edge case in that convention; do not "fix" it without a wider audit
+/// of how upstream writers emit CW boundary arcs.)
+///
+/// VERIFIED against real AutoCAD output (KSR-039 DWG, CW arc edges with
+/// line neighbours): the stored angles of a `ccw = false` arc edge are
+/// MIRRORED — the true point is `center + r·(cos(TAU-θ), sin(TAU-θ))`.
+/// Sampling the mirrored parameter directly (this function's flip) gives
+/// exact endpoint continuity with the adjacent edges (Δ = 0.000000),
+/// while interpreting them as true angles lands tens of units away. Any
+/// code that *produces* CW boundary arcs (mirror transforms, explode)
+/// must therefore store mirrored angles, not geometric ones.
+pub fn arc_signed_span(start: f64, end: f64, ccw: bool) -> (f64, f64) {
+    const TAU: f64 = std::f64::consts::TAU;
+    let (sa, ea) = if ccw { (start, end) } else { (TAU - start, TAU - end) };
+    (sa, ea - sa)
+}
+
+/// Segment count for an arc, targeting `chord_tol_world` chord-height
+/// error in world units. Floor 8, cap 512.
+///
+/// Two production callers:
+/// - hatch fill polygon (built at load / on edit) passes a radius-
+///   relative tol via [`fill_chord_tol`] — ~0.1% radius, zoom-free so
+///   the polygon stays sharp at extreme zoom-in without re-tessellation.
+/// - hatch wire outline (re-tessellated every frame inside the render
+///   scope) passes a zoom-adaptive tol via [`wire_chord_tol`] — pulls
+///   from the per-frame override set by `Scene::wires_for_block` so
+///   far-out arcs collapse to a handful of segments.
+pub fn arc_segments(radius: f64, span_abs: f64, chord_tol_world: f64) -> u32 {
+    if span_abs < 1e-9 || radius < 1e-9 {
+        return 1;
+    }
+    let tol = chord_tol_world.max(1e-9).min(radius * 0.99);
+    // θ where r * (1 - cos(θ/2)) = tol  →  θ = 2 * acos(1 - tol/r).
+    let max_step = (2.0 * (1.0 - tol / radius).acos()).max(1e-6);
+    ((span_abs / max_step).ceil() as u32).clamp(8, 512)
+}
+
+/// Chord tolerance for the load-time fill polygon: 0.1% of radius,
+/// floor 1 µm so degenerate radii still produce a workable count.
+pub fn fill_chord_tol(radius: f64) -> f64 {
+    (radius * 0.001).max(1e-6)
+}
+
+/// Chord tolerance for the per-frame wire outline: pulls the active
+/// `truck_tess::set_curve_tol_override` value (Scene sets it to
+/// `world_per_pixel × 0.5` so curves stay at ~half-pixel chord error at
+/// the current zoom). When no override is active (snap / hit-test
+/// passes, load-time builds), falls back to [`fill_chord_tol`] so we
+/// never under-sample.
+pub fn wire_chord_tol(radius: f64) -> f64 {
+    match crate::scene::convert::truck_tess::active_curve_tol() {
+        Some(t) => t.min(fill_chord_tol(radius)).max(1e-9),
+        None => fill_chord_tol(radius),
+    }
+}
+
+// ── Colour helper ──────────────────────────────────────────────────────────
+
+/// Convert an acadrust Color (ACI index or true-color) to a GPU RGBA value.
+pub fn aci_to_rgba(color: &AcadColor) -> [f32; 4] {
+    if let Some((r, g, b)) = color.rgb() {
+        [r as f32 / 255.0, g as f32 / 255.0, b as f32 / 255.0, 1.0]
+    } else {
+        WireModel::WHITE
+    }
+}