test(3d): screenshot-baseline TDD suite for the 3D viewer OpenGL->WebGL port — 47 native goldens + red-state WebGL harness

tests/3d-regression mirrors the gal-regression pattern at renderer scale:
shared C++ scenarios call real KiCad 3D-viewer code (opengl_utils, display
lists + DrawCulled stencil subtraction, MODEL_3D VBOs, private generators via
a rob-template accessor, and full reload()+Redraw() composites over a
synthetic BOARD_ADAPTER). A native macOS harness renders them on real OpenGL
into 47 committed goldens (bit-deterministic, FBO capture); the wasm harness
compiles the same TUs against wasm/stubs/gl_ffp_stub.c no-ops so every
scenario renders blank — the TDD red state (parity meter: 47/47 changed).
Comparisons use the CI pixelmatch engine via the new generic compare-dirs.ts
(floors.json levels; manifest.json cmp-guards registry drift).

Documents an upstream bug: appendPostMachiningGeometry's countersink path
adds middle quads without normals, silently erasing the walls of any
display list it is batched into (3d-post-machining.png keeps the lists
separate to record it).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/3d-regression/floors.json

@@ -0,0 +1,14 @@
+{
+  "native-self": {
+    "default": { "changedRatio": 0.001, "meanChannelGuard": 2.0 },
+    "overrides": {}
+  },
+  "webgl-vs-native": {
+    "default": { "changedRatio": 0.02, "meanChannelGuard": 4.0 },
+    "overrides": {}
+  },
+  "webgl-self": {
+    "default": { "changedRatio": 0.005, "meanChannelGuard": 2.0 },
+    "overrides": {}
+  }
+}