fix(renderer): derive SECTION_2D_UNIFORM_SLOT_COUNT from the slot index

SECTION_2D_UNIFORM_SLOT_COUNT sized the shared section-2D-overlay
uniform buffer with a hand-written 6, while SECTION_2D_UNIFORM_SLOT_INDEX
maps each draw site to its slot. Nothing tied the two together: a new
key added to the index type-checks fine (a property access needs the
key to compile) but the count doesn't grow with it, so the buffer stays
one slot short and the new draw site's bind-group offset lands one
record past the end.

Derive the count from the index instead, and pin the two equal with a
test so a future divergence fails a test rather than only surfacing as
a WebGPU validation error on the new site.

Refs #3342

Author: BIMvoice

.changeset/section-2d-slot-count-derived.md

@@ -0,0 +1,5 @@
+---
+'@ifc-lite/renderer': patch
+---
+
+Derive `SECTION_2D_UNIFORM_SLOT_COUNT` from `Object.keys(SECTION_2D_UNIFORM_SLOT_INDEX).length` instead of a hand-written `6`, so adding a draw site to the index can no longer leave the shared uniform buffer one slot short of what the index addresses. A test pins the two values equal so a future divergence fails loudly instead of only showing up as a WebGPU bind-group validation error on the new draw site.

packages/renderer/src/section-2d-overlay-lifecycle.test.ts

@@ -334,6 +334,15 @@ describe('Section2DOverlayRenderer: dispose releases EVERY family (#1277 leak)',
   });
 });
 
+describe('SECTION_2D_UNIFORM_SLOT_COUNT (#3342)', () => {
+  it('equals the number of entries in SECTION_2D_UNIFORM_SLOT_INDEX', () => {
+    assert.strictEqual(
+      SECTION_2D_UNIFORM_SLOT_COUNT,
+      Object.keys(SECTION_2D_UNIFORM_SLOT_INDEX).length,
+    );
+  });
+});
+
 describe('Section2DOverlayRenderer: shared uniform buffer', () => {
   function lastWrite(writes: Array<{ data: Float32Array }>): Float32Array {
     assert.ok(writes.length > 0, 'expected a uniform write');

packages/renderer/src/shaders/section-2d-overlay.wgsl.ts

@@ -80,8 +80,8 @@ export const SECTION_2D_UNIFORM_SLOT_INDEX = {
   clashBox: 5,
 } as const;
 
-/** How many uniform records the shared buffer holds. */
-export const SECTION_2D_UNIFORM_SLOT_COUNT = 6;
+/** How many uniform records the shared buffer holds — one per entry above. */
+export const SECTION_2D_UNIFORM_SLOT_COUNT = Object.keys(SECTION_2D_UNIFORM_SLOT_INDEX).length;
 
 /**
  * Byte stride between uniform slots for `device`.