rendering: reuse retained geometry resources

[tritao]

Builds on #68.

Purpose

Reusing an unchanged frame helps only while that exact draw list survives. A
rebuild could still repack identical geometry and make the OpenGL backend
recreate or revalidate resources for every repeated occurrence.

This PR gives geometry stable draw-list resource identity. Commands refer to
shared geometry resources, allowing repeated shapes and rebuilt frames to
reuse the same packed data and backend allocation when their source is still
valid.

What changed

• Add draw-list-owned geometry resources and stable command handles.
• Route backend geometry caching through those handles.
• Let retained picking and subelement ranges follow the shared geometry
  resource rather than duplicate command-local ownership.
• Add direct triangular SoFaceSet emission to avoid unnecessary generic
  primitive reconstruction.
• Reuse stable, non-textured overall-material face-set sources conservatively
  across occurrences and rebuilt frames.

Safety and scope

Resource reuse is based on explicit source identity and revision. Cases whose
effective geometry cannot yet be proven stable continue through the existing
capture path. Broader textured and per-vertex-material source reuse remains
outside this layer.

Validation

• Forced-rebuild workloads compare resource reuse with the preceding layer.
• Retained IR tests cover resource ownership and command references.
• Vertex colors, picking identity, and subelement ranges remain correct when
  geometry resources are shared.

[tritao]

Added stable scene-texture and explicit texture-coordinate source identity. On the accelerated Intel OpenGL runner, the 50k core forced rebuild improved from 1106.8 ms to 319.3 ms; backend resource preparation improved from 645.2 ms to 76.2 ms. Procedural texture-coordinate functions remain conservatively excluded. Full rebuild and all 37 tests pass.

[tritao]

Added conservative PER_VERTEX color-source reuse with content-based color and opacity revision tracking. The 50k core forced rebuild improved from 319.3 ms to 233.0 ms; backend resource preparation improved from 76.2 ms to 4.3 ms. Packed-color and underspecified diffuse arrays remain excluded. Full rebuild and all 37 tests pass.

[tritao]

Added retained construction attribution in c8ce046.

50k feature-rich forced rebuild (5 samples, local hardware):

Phase	Core median	Compatibility median
Total draw-list construction	147.03 ms	147.91 ms
Primitive generation	11.99 ms	12.03 ms
Geometry packing	0.004 ms	0.004 ms
Command emission/state capture/path retention	55.12 ms	55.35 ms
Residual traversal/state/orchestration	79.92 ms	80.53 ms
Backend resource preparation	4.30 ms	4.23 ms
Backend command execution	81.21 ms	78.44 ms
Total CPU render	238.66 ms	236.93 ms

This confirms geometry generation/packing is no longer the rebuild bottleneck. The next candidates are command/state capture and the residual traversal/state/orchestration cost. JSON schema is now version 6. Full local suite: 37/37 passed.