fix(examples): redesign bmesh-gear as a brass gear in a bolted gear train

The hero was a single flat brass gear leaning on a wedge. The checked gear
now sits in a small gear train on a dark painted steel backplate over a
walnut plinth, meshing with a blued 8-tooth pinion and a 22-tooth gunmetal
wheel with a spoked web. Each gear has a hub boss, steel shaft, washer and
hex nut.

- The companions are render-only meshes built from the same
  four-verts-per-tooth profile and pitched to the checked gear, so all
  three share one circular pitch.
- Each companion is spun so a gap sits on the line of centres, facing a
  tooth of the checked gear.
- A render-only Bevel modifier chamfers the edges after check() has run.
- The checked mesh, its machined-brass material and the checks are
  unchanged. The wedge-staging constants are removed.

Evidence (live runs, local):
- .scratch/blender-5.2.1-windows-x64/blender.exe (Blender 5.2.1 LTS):
  - check-only exits 0: teeth=14, verts=112, edges=168, faces=58,
    watertight=True.
  - --no-extrude exits 3: "topology (56, 56, 1) != expected (112, 168, 58)".
- .scratch/blender-4.5.11-windows-x64/blender.exe (Blender 4.5.11 LTS):
  check-only exits 0 with the same counts, and --output renders with
  framing_ok.
- Framing: fill x=0.653, y=0.883. Margins: left 0.172, right 0.175,
  bottom 0.056, top 0.061.
- Contact sheet (docs/gallery/contact-sheets/bmesh-gear-contact-sheet.webp):

  | tile                      | luma  | stage | warmth | sat   |
  |---------------------------|-------|-------|--------|-------|
  | bmesh-gear (candidate)    | 0.237 | 0.252 | +0.092 | 0.153 |
  | armature-bend             | 0.243 | 0.192 | -0.144 | 0.237 |
  | damped-track-aim          | 0.237 | 0.218 | +0.105 | 0.178 |

  The stage reads brighter because the lit backplate reaches the top
  border.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Author: TMHSDigital

README.md

@@ -357,7 +357,7 @@ Gallery still is a sky-lit obelisk diptych (8° dusk | 55° midday) so the contr
 <table>
 <tr>
 <td width="46%" valign="middle">
-<a href="examples/bmesh-gear/"><img src="examples/bmesh-gear/preview.webp" alt="Bmesh gear: a brass 14-tooth gear leaning back on a dark display wedge, its lathe-turned face catching a soft highlight that fans from the centre and its hobbed tooth flanks lit against the stage" /></a>
+<a href="examples/bmesh-gear/"><img src="examples/bmesh-gear/preview.webp" alt="Bmesh gear: a brass 14-tooth gear in a small gear train, meshing with a blued 8-tooth pinion and a spoked 22-tooth gunmetal wheel, each bolted through a hub boss to a dark steel backplate on a walnut plinth" /></a>
 </td>
 <td valign="middle">
 

docs/gallery/assets/bmesh-gear-hero.webp

@@ -11,23 +11,23 @@ check asserts the closed-form counts — verts = 2 × (4 × teeth), faces = side
 edges = 3 × profile — and that the result is watertight (every edge borders exactly two
 faces). If an op leaks geometry or a face fails to close, the math catches it.
 
-The still leans the gear back on a matte inclined display wedge. Both
-contacts are derived from the posed mesh: the lowest back-cap vertex sits 3 mm
-into the floor, and the wedge's slope lies in the back-cap plane, so the lean
-is carried rather than held in the air.
-
-The finish follows how a gear blank is actually cut, and the render path
-changes only the material and lights — the mesh the check counts is untouched.
-The caps are lathe-faced: turning marks run concentric about the gear's own
-axis (object coordinates, so they centre on that axis rather than on a
-bounding-box corner), far finer than a pixel, so they read only as a narrow
-satin roughness band and a whisper of bump — the soft highlight that fans
-from the centre. The tooth flanks are hobbed, a rougher finish that spreads
-the light across every facet. The key highlight on the face is a softbox
-placed on the camera ray's reflection about the front cap, and a low bounce
-card gives the downward flanks something to mirror. There is no bore: the
-closed-form topology check counts exactly two rings and two caps, so the
-gear stays a solid blank rather than faking a hole in the shader.
+The still mounts the checked gear in a small gear train on a painted steel
+backplate over a walnut plinth: a blued 8-tooth pinion above right and a
+22-tooth gunmetal wheel with a spoked web below left, each on a hub boss,
+steel shaft, washer and hex nut. The companions are render-only meshes built
+from the same four-verts-per-tooth profile, pitched to the checked gear
+(pitch radius midway between root and tip, so all three share one circular
+pitch), and each is spun so a gap sits on the line of centres facing a tooth
+of the checked gear. A render-only Bevel modifier chamfers the edges after
+the check has run, so the mesh the check counts is untouched.
+
+The checked gear keeps its machined finish: lathe-faced caps whose turning
+marks run concentric about the gear's own axis (object coordinates), far
+finer than a pixel, so they read only as a satin sheen, and rougher hobbed
+tooth flanks. A softbox above and left of the camera gives the brass face
+one warm reflection. There is no bore in the checked gear: the closed-form
+topology check counts exactly two rings and two caps, so the hub boss and nut
+sit on its face rather than faking a hole.
 
 ## Run