LOD: fige les jumeaux de fente (slit twins) + blend UV linéaire (le fold est supprimé)

- weld: Δ UV = 0.5 exact n'est plus soudé (ambigu: fente à sa plus large
  vs saut légitime — la colonne u=1 du cône vs le chart disque du cap
  tombait exactement dessus et mélangeait les charts)
- collapse: les UV se blendent linéairement. Le fold par coordonnée
  (Δ entier → 0) figeait l'UV du sommet sur les vertices de base
  (bande de rayures, signalée par l'utilisateur) — il était inutile:
  les jumeaux de fente sont gelés, aucun repli ne traverse la fente
- welded renvoie un struct Welded (clippy)
- test cône: dual-chart (fan latéral bilinéaire + disque du cap),
  surface latérale r = λ; test seam/span réécrit selon la sémantique finale
- doc: gpu-driven.md, ARCHI_CPU_GPU.md (box LOD), DRAFT.md (D10) alignés
  sur la sémantique finale (gel des jumeaux, blend linéaire, seuil strict)
- AGENTS.md: gotcha 'ne jamais folder un saut entier de tuile'

102 tests passent, demo lance et rend sans erreur.

docs/user/gpu-driven.md

@@ -98,15 +98,20 @@ CPU):
    cheapest-first; an interior collapse merges both incident triangles (−2 faces) and remaps the
    neighbours — no new face, so a **seam-free mesh stays closed** (no holes, no non-manifold
    "books") and a boundary collapse removes one face; duplicate corners are welded **aware of
-   their attributes** (relative position tolerance 1e-6, merged only when the UVs are within half
-   a tile and the normals within ~25° — a seam or a hard edge stays a separate corner, so a
-   collapse can never cross a texture chart or a shading break; on a mesh with a UV seam the
-   seam's lips are then *protected* — their faces never collapse while interior ones remain — so
-   the surface stays geometrically complete); a survivor **moved** by a collapse gets its
-   UV/color/normal **interpolated** between the collapsed endpoints (same λ as its new position —
-   texture and shading stay attached to the surface and coarsen smoothly across levels, never
-   across a UV seam; normals are **inherited from the source, never recomputed**, so the lighting
-   is identical to level 0 whatever the source's winding), and the
+   their attributes** (relative position tolerance 1e-6, merged only when the UVs are strictly
+   less than half a tile apart on both coordinates — an offset of exactly ½ is ambiguous: a wrap
+   seam at its widest or a legitimate half-tile jump — and the normals within ~25°; a seam or a
+   hard edge therefore stays a separate corner, and the weld *records* the integer-apart pairs it
+   refused); on a mesh with a UV seam those **seam twins are frozen** — every edge touching one
+   is excluded from the collapse queue — so the zero-width slit stays closed at every level, and
+   the rim protection (no boundary collapse while any interior edge remains) keeps the surface
+   geometrically complete; a survivor **moved** by a collapse gets its UV/color/normal
+   **blended linearly** between the collapsed endpoints (same λ as its new position — the chart
+   is bilinear, so the blend is the exact chart value at the new point: texture and shading stay
+   attached to the surface and coarsen smoothly across levels, and a seam is never crossed because
+   its twins are frozen, not because a blend is rejected; normals are **inherited from the
+   source, never recomputed**, so the lighting is identical to level 0 whatever the source's
+   winding), 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