LOD: quadric edge collapse (Garland-Heckbert) replaces Rule A

Rule A (area-sorted triangle removal) left holes and open boundaries
on closed meshes (visible artifacts when zoomed far out). The decimator
is now a proper quadric edge collapse:

- Collapse: welded u32 topology, per-vertex quadrics (accumulated
  incident face planes), edge cost = quadric error at the optimal
  point (clamped to the segment) + edge length, BinaryHeap with a
  custom Ord (f32 is not Ord; inverted compare, NaN-safe).
- Standard GH semantics WITHOUT the new face: both incident pair faces
  degenerate and are removed; neighbouring faces remap and sweep over
  the region. Preserves the Euler characteristic and closedness (no
  holes, no books, no duplicate faces); interior collapse = -2 faces,
  boundary = -1. Guards: non-manifold edge (>2 faces) or a fold
  (duplicate sorted triple) rejects the collapse.
- welded(): fuzzy welding (1e-6 relative tolerance, grid + 27-
  neighbour broad phase, exact verify) - trig-generated seams differ
  by ~1e-16, exact-bit welding missed them.
- Root causes fixed along the way: dead face slots are never reused
  (stale edge/vface entries), vfaces updated on remap, degenerate
  faces dropped, best-effort target (granularity -2/-1 can land 1-2
  off; soft cap, deterministic).
- Docs: DRAFT D10, ROADMAP 4.3, ARCHI_CPU_GPU, gpu-driven, lib
  README, mesh/scene/demo comments - 'greedy decimation / Rule A'
  replaced by 'quadric edge collapse'.
- lod.rs test: deprecated glam perspective alias -> explicit
  glam::camera::rh::proj::opengl::perspective.

cargo test --workspace: 100 passed (94 lib + 3 wgsl + 3 integration),
0 failed; demo runs clean with LOD on.

docs/user/gpu-driven.md

@@ -93,9 +93,12 @@ LOD is a **CPU-decided, GPU-executed** split (the one deliberate per-entity deci
 CPU):
 
 1. **Setup (once per mesh).** Each mesh can carry up to 4 levels. Levels 1..3 are generated
-   automatically from level 0 by greedy decimation (`Geometry::generate_lod_levels`): the smallest
-   triangles are removed first (no edge map, no crease handling — a subset of the faces of a valid
-   mesh is valid), duplicate corners are welded, and the levels are **packed into the mesh's single
+   automatically from level 0 by **quadric edge collapse** (Garland–Heckbert,
+   `Geometry::generate_lod_levels`): edges are ranked by quadric error and collapsed
+   cheapest-first; an interior collapse merges both incident triangles (−2 faces) and remaps the
+   neighbours — no new face, so a **closed mesh stays closed** (no holes, no non-manifold
+   "books"), a boundary collapse removes one face; duplicate corners are welded (relative
+   tolerance 1e-6), and the levels are **packed into the mesh's single
    vertex/index buffers** (see the constraint below). Level 0 is always your exact geometry.
 2. **Per frame (CPU).** For each entity, the bounding sphere used by culling is projected to screen
    pixels (its *perceived size*); that radius picks a level with **asymmetric hysteresis** — going