chore(scene): drop INSERT_SUB_LIMIT guard from fallback explode path

The 5,000 sub-entity guard was added when an xref block with ~74k subs
froze the UI on first render. The block_cache primary path (def72e4)
made that path unreachable for typical Inserts — the guard now only
gated the rare legacy-explode fallback, where it hid otherwise-renderable
geometry as an insertion marker.

Remove the constant and its early-return; the fallback path now matches
the primary path's "render everything" behaviour. Also drop the
RENDERING_OPTIMIZATION.md "TEMPORARY" section that tracked this guard.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

RENDERING_OPTIMIZATION.md

@@ -112,43 +112,3 @@ Phase 4.2  Hi-Z occlusion                   high complexity, 3D only, last
 - **Phase 2 target:** Pan/zoom frame cost O(visible) not O(total).
 - **Phase 4 target:** GPU-cull overhead < 0.5 ms for 1M entity scene.
 
----
-
-## TEMPORARY: Insert sub-entity count limit (revert when better fix lands)
-
-`tessellate_entity` in `src/scene/mod.rs` has a guard that short-circuits any
-Insert whose referenced block contains more than `INSERT_SUB_LIMIT` (currently
-5,000) sub-entities. The Insert is rendered as an insertion marker only — no
-explode, no sub tessellation.
-
-**Why this exists:** A real-world DWG was found where an xref block contained
-~74k sub-entities. Opening the file froze the UI on the first render because
-`tessellate_entity` exploded the block and serially style-resolved every
-sub-entity (and any single bad sub could hang tessellation forever). The guard
-made the file openable.
-
-**Why it should go away:** The limit hides legitimate geometry. Large xrefs are
-common in civil/infrastructure drawings — the user shouldn't have to see them
-as crosses.
-
-**Proper fixes that would let us drop this guard:**
-1. ~~Per-block tessellation cache~~ — **LANDED** (`def72e4`): block defns are
-   tessellated once and per-Insert reuses the cached wires via
-   `block_cache::expand_insert`. The guard now only fires on the legacy
-   fallback path; consider lowering or removing it.
-2. Lazy / background Insert tessellation — show marker first, fill in detail
-   on a background thread, redraw when ready.
-3. Per-sub-entity hardening — find why a single sub can hang tessellation
-   (likely Spline NURBS evaluation or malformed boundary edge) and fix the
-   root cause; combine with sub-count budget per frame instead of a hard cap.
-4. xref attach metadata respected — DWG carries an "unloaded" state for xref
-   blocks; honour it instead of always inlining xref content on open.
-   (Attempted in `e88a946`, reverted in `0996b75` — the on-disk bit was
-   unreliable as a Loaded/Unloaded signal.)
-
-When any of (2)–(3) lands, delete the `INSERT_SUB_LIMIT` block in
-`tessellate_entity` (search for `INSERT_SUB_LIMIT`).
-
-Related companion fix in `src/app/update.rs`: after `resolve_xrefs`, a second
-`purge_corrupt_entities` pass runs against the newly inlined xref content.
-That one is not a hack — it should stay even after the Insert guard goes.

src/scene/mod.rs

@@ -4344,18 +4344,13 @@ fn tessellate_entity(
             }
         }
 
-        // Cache miss / unavailable: fall back to the original explode path,
-        // but keep the safety bail-out for pathologically large blocks so
-        // the UI doesn't hang on broken or unbounded sub-entity counts.
-        const INSERT_SUB_LIMIT: usize = 5_000;
+        // Cache miss / unavailable: fall back to the original explode path.
+        // The block_cache primary path covers all typical Inserts; this
+        // branch only fires for pathological cache failures.
         let br = document.block_records.get(&ins.block_name);
-        let sub_count = br.map(|br| br.entity_handles.len()).unwrap_or(0);
         let is_xref = br
             .map(|br| br.flags.is_xref || br.flags.is_xref_overlay)
             .unwrap_or(false);
-        if sub_count > INSERT_SUB_LIMIT {
-            return vec![marker];
-        }
         let mut wires: Vec<WireModel> = ins
             .explode_from_document(document)
             .iter()