feat: AI asset pipeline track, Phase 3 (pipeline template)

[TMHSDigital]

Summary

Third template: templates/ai-asset-pipeline-template/. Headless GLB in, engine-ready LOD set plus optional collider out. Composes ai-mesh-cleanup, lod-decimate-chain, collision-hull-proxy, and engine-export-presets by duplicating the named snippets (templates are not a package; same convention as snippets/lod_chain.py).

Does not touch VERSION, CHANGELOG.md, release.yml, the smoke version matrix, examples, or LICENSE.

Follow-up: #134 (add 5.1 to the PR smoke matrix). This PR does not change that matrix.

Part A (leftover branch, done locally)

feat/property-delete-and-usd-witnesses at b5cac0b was squash-merged as PR #126 (MERGED 2026-09-09, merge SHA 764c90f). Content is on main (examples/cross-version-property-delete/, examples/usd-export-evaluation-mode/). Local leftover deleted with git branch -D. Not unmerged work.

Exit-code convention (inspection, then live-run of the codes)

There is no repo-wide exit-code table. examples/export-preset-axis/ exit 9 is example-local.

This template conforms to templates/headless-batch-script-template/script.py and its README: 0 success, 2+ distinct failure modes, argparse usage also exits 2. Documented in templates/ai-asset-pipeline-template/README.md and the pipeline.py header.

Code	Meaning
0	Success
2	Input missing, or argparse rejected flags (including unsupported --preset)
3	Input is not a readable GLB
4	--lod-budgets missing, non-positive, or not strictly decreasing
5	Import produced no mesh (not canaried with a committed GLB; inspection + code path)
6	outdir is not a directory, or export wrote nothing / raised

Test plan

• Validate + drift-check green on this PR
• Blender 5.2 smoke green (CI)
• Blender 4.5 smoke green (CI)
• Socket checks not pending/failing
• 5.1 is not in the PR matrix; local result below is not CI evidence

Evidence labels

Live-run-proven (local Windows)

Binaries (each blender.exe --version):

• .scratch/blender-4.5.11-windows-x64/blender.exe → 4.5.11 LTS (4db51e9d1e1e)
• .scratch/blender-5.1.2-windows-x64/blender.exe → 5.1.2 (ec6e62d40fa9)
• .scratch/blender-5.2.1-windows-x64/blender.exe → 5.2.1 LTS (9e2066aef7ef)

Happy path: runtime-generated UV sphere fixture (u=48,v=24), then pipeline. Identical counts on all three series: evaluated_tris=2208, LOD budgets 1024 / 256 / 64 exact, exit 0.

Binary	Preset	Collider	Exit
5.2.1	unity	convex (1106 verts)	0
4.5.11	godot	box (8 verts)	0
5.1.2	unreal	box (8 verts)	0
5.2.1	unreal	box (8 verts)	0

5.1.2 result is local live-run only. blender-smoke.yml runs 4.5 and 5.2 on PRs; 5.1 is Monday cron. Absence of a 5.1 PR job is not a pass. See #134.

B4 canaries (5.2.1 only, verbatim)

===== missing-input =====
ERROR: input file missing: E:\Blender-Developer-Tools\.scratch\pipeline-test\missing.glb
EXIT:2
===== not-glb =====
ERROR: input is not a readable GLB: E:\Blender-Developer-Tools\.scratch\pipeline-test\not.glb
EXIT:3
===== bad-preset =====
usage: blender.exe [-h] --input INPUT --outdir OUTDIR
                   --preset {unity,godot,unreal} [--lod-budgets LOD_BUDGETS]
                   [--collider {convex,box,none}] [--draco]
blender.exe: error: argument --preset: invalid choice: 'source' (choose from unity, godot, unreal)
EXIT:2
===== budgets-nonmonotonic =====
ERROR: --lod-budgets must be comma-separated positive ints, strictly decreasing
EXIT:4
===== budgets-nonpositive =====
ERROR: --lod-budgets must be comma-separated positive ints, strictly decreasing
EXIT:4
===== export-outdir-not-dir =====
ERROR: outdir is not a directory: E:\Blender-Developer-Tools\.scratch\pipeline-test\notdir
EXIT:6

Canaries were gitignored under .scratch/pipeline-test/ and were not committed. Exit 5 (no mesh after import) was not fired with a committed asset; the path is inspection-only.

Inspection-only

• Inventory counts (15 / 9 / 3 / 24 / 54) vs filesystem and plugin.json
• Roadmap template sub-track marked delivered; agent bridge still unpinned
• Smoke YAML steps mirror the existing headless glTF template (fixture, exit 0 + magic, one failure exit)
• No 4.5 / 5.1 / 5.2 output divergence on the measured triangle counts
• Skills/rules/snippets were not modified

Out of scope (unchanged)

Live-session agent bridge. LICENSE / #131. All 54 examples. VERSION / CHANGELOG.md / release.yml. Smoke version matrix. Vendor integrations. New examples.