fix(examples): redesign curve-bevel-arc as a horseshoe magnet with field-line filings

[TMHSDigital]

What changed and why

The gallery review called the old hero programmer art: one plain rose tube lying on the floor. This PR redesigns the render. The check does not change.

The checked curve (8 Bezier points, bevel_depth 0.15, use_fill_caps) is now staged as a round-bar horseshoe magnet lying on the floor with its open end toward the camera:

• Red enamel with ground-steel pole ends. This is one material keyed on object-space Y, so the magnet is still a single curve datablock.
• The two filled caps are the pole faces. They are the use_fill_caps witness, and they face the camera. An uncapped build would show hollow pipe ends in exactly that spot.
• Paper clips (teal, amber) are beveled Bezier curves too. They use explicit FREE handles, with straight legs and quarter-circle bends at the 4/3·tan(π/8) handle length. Each clip lies along the field line that leaves its pole.
• Iron filings follow the field lines. For two opposite line poles, the field lines are exact circular arcs through both pole faces.
• Steel bearings sit on one pole shoe.

Shared-tooling change (small, additive):

• examples/gallery_asset_quality.py now counts curve objects as hero parts. It checks naming, materials, and edge90; edge90 runs on the depsgraph-evaluated tessellation, which is freed with to_mesh_clear().
• scripts/asset_sheet_panel.py now keeps curve objects.
• Before this change, a curve-built asset was silently skipped by both.
• No existing caller passes a curve object. prop-origin-transform's floors are identical before and after: materials n=7 dominant 0.50, edge90 446/5973.

Before / after

• Contact sheet: docs/gallery/contact-sheets/curve-bevel-arc-contact-sheet.webp
• Asset sheet: docs/gallery/asset-sheets/curve-bevel-arc.webp
• The old hero (a single magenta tube) is in the parent commit's docs/gallery/assets/curve-bevel-arc-hero.webp.

Gate numbers (5.2.1 LTS)

• Framing: fill x=0.759, y=0.478 (band 0.70–0.90). Margins: L 0.153, R 0.059, B 0.150, T 0.078. framing_ok.

• Asset quality:

  • naming ok
  • materials n=4, dominant MagnetEnamelAndPoles@0.25
  • edge90 0/7686
  • compactness 178 (informational)
  • aq_ok

• Contact sheet (luma / stage / warmth / sat):

  Tile	Luma	Stage	Warmth	Sat
  candidate	0.2286	0.2191	+0.025	0.182
  armature-bend	0.2787	0.2132	−0.428	0.144
  damped-track-aim	0.1511	0.1448	+0.277	0.231
  bmesh-gear	0.2358	0.1772	+0.303	0.250

  • Stage darkness: pass. It is inside the calibration range and matches armature-bend.
  • Wedge warmth: marginal. It is warm but weaker than gear and aim; the pool sits upper right on the floor and wall.
  • Subject fill: pass.
  • Saturation: pass. It is between the calibration tiles.
  • Thumbnail legibility: pass. At 427×240 it reads as a red horseshoe magnet with silver poles and a filings field.

• Asset sheet verdict: borderline, reviewer call. The finish holds up next to the references: two-tone enamel/steel, clean bevels, and designed clips. By part count and silhouette it is the simplest object in the lineup, because a horseshoe magnet is a simple product. It is not sloppier than the references, but it is the least complex.

• Hero drift: measure_hero_drift.py --only curve-bevel-arc: mean_abs 0.0036, verdict matches.

Proven by live run

Local binaries: E:\Blender-Developer-Tools\.scratch\blender-{5.2.1,5.1.2,4.5.11}-windows-x64\blender.exe, reporting Blender 5.2.1 LTS, Blender 5.1.2, Blender 4.5.11 LTS.

• Check-only path: exit 0 on all three with points=8 bevel=0.15 caps=True eval_verts=1044 eval_faces=1028 z=-0.000..0.300 x_span=3.292. tests/smoke/run_example.py gives [PASS] on all three.
• Falsifier --no-caps: exit 6 (use_fill_caps is False) on all three.
• Render path: framing and asset-quality gates pass on all three binaries. The hero was rendered with 5.2.1 via scripts/render_hero.py.
• Helper falsifier (new curve coverage): I temporarily named the magnet object BezierCurve. The render path exited 11 with aq_naming default_names=['BezierCurve'] FAIL, then I restored the name. With the old helper the curve was invisible to the floors, so this probe could not have failed.
• Gallery: build_gallery.py was re-run after rebasing on main. I read the generated <img alt> in docs/gallery/index.html and docs/gallery/curve-bevel-arc/index.html and both show the new sentence.

Established by inspection only

• The claim that an uncapped build would show hollow pole faces. The check exits 6 before any render, so no uncapped still was produced.
• The field-line geometry. The arcs are the analytic solution; no field was simulated.

🤖 Generated with Claude Code