feat: AI asset pipeline track, Phase 1 - #132

Summary

Phase 1 of the provider-agnostic AI asset pipeline track (GLB in, engine-ready out). This repo does not generate meshes. No generation vendor is referenced in the new files.

• ROADMAP: four-sub-track track (cleanup skills, engine export presets, headless template, live-session spike). Upcoming, unpinned, matching existing cadence.
• Skill ai-mesh-cleanup: ordered cleanup sequence. Composes depsgraph-and-evaluated-data and mesh-editing-and-bmesh; does not replace them. Existing examples (lod-decimate-chain, collision-hull-proxy, unapplied-scale-gltf, prop-origin-transform, mesh-hygiene-audit) remain witnesses of individual contracts.
• Four snippets: decimate_to_budget.py, convex_hull_collider.py, lod_chain.py (helper duplicated; snippets are not a package), gltf_draco_export.py.
• Two rules: validate-imported-mesh-scale, no-unapplied-modifiers-on-export.
• CI static gate: tests/check_import_export_rules.py scans snippets/ and templates/. examples/ is excluded because pathology witnesses are intentional.

Out of scope (untouched): ai-asset-pipeline-template/, live-session bridge, LICENSE, gallery/site.json, new examples/ entries.

calc_loop_triangles

Live-run-proven (docs + runtime): Mesh.calc_loop_triangles() is still required before reading mesh.loop_triangles on 4.5 LTS, 5.1, and 5.2. Tessellation is not implicit. No hasattr guard; the call is unconditional, same as examples/lod-decimate-chain/ and skills/depsgraph-and-evaluated-data.

Verified against:

• https://docs.blender.org/api/5.1/bpy.types.Mesh.html#bpy.types.Mesh.calc_loop_triangles (Calculate loop triangle tessellation (supports editmode too))
• https://docs.blender.org/api/5.1/bmesh.html (tessellation "needs to be called explicitly")
• https://docs.blender.org/api/4.5/bpy.types.Mesh.html#bpy.types.Mesh.calc_loop_triangles
• Bundled current Mesh RNA (5.2) still lists calc_loop_triangles().

glTF axis: operator RNA exposes export_yup, not FBX axis_forward / axis_up. The snippet passes export_yup explicitly. Inspection-only of RNA via the 5.2 bundled docs; live export with export_yup=True and Draco live-run-proven below.

Test plan

• python tests/check_import_export_rules.py passes on snippets and templates
• Canary fixtures fail the gate (output below), then removed (never committed; lived in gitignored .scratch/)
• Snippet helpers probed headless on local binaries (not the full 53-example catalog)
• blender-smoke 5.2 and 4.5 jobs on this PR (CI)
• validate.yml / validate-counts / validate-manifest / drift-check on this PR (CI)

Evidence

Local binaries (live-run-proven)

Probe covered: evaluated triangle count via evaluated_get / to_mesh / to_mesh_clear / calc_loop_triangles; DECIMATE COLLAPSE ratio clamp; under-budget returns None; convex hull collider copies matrix_world; LOD copies do not share the source mesh datablock; Draco glTF write with use_selection=True, export_yup=True, export_apply=True.

Full blender-smoke catalog (53 examples) was not run locally. That claim is inspection-only until CI.

Task 5 canary (live-run-proven)

Command:

python tests/check_import_export_rules.py .scratch/canary_import_scale.py .scratch/canary_unevaluated_export.py

Exit 1. stderr:

ERROR: .scratch/canary_import_scale.py: import_scene gltf/fbx plus mesh work without transform_apply and a unit-scale check (unit_settings, scale_length, or global_scale)
ERROR: .scratch/canary_unevaluated_export.py: export with modifiers.new but no export_apply=True, evaluation_mode, or modifier_apply

Clean tree (no extra paths) exits 0: import/export anti-pattern checks passed.

The two .mdc files are Cursor rules; CI enforces the same anti-patterns via the static scanner. A gate that has only ever passed is not trusted; both canaries failed as required.

License tracking

#131: ND terms vs documented snippet copy-and-adapt. LICENSE is not edited in this PR.