feat: AI asset pipeline track, Phase 2 (engine export presets)

[TMHSDigital]

Summary

Phase 2 of the AI asset pipeline track: the engine export layer that Phase 3's templates/ai-asset-pipeline-template/ will call. One skill, three snippets, one rule plus harness extension, one example.

• Skill engine-export-presets composes ai-mesh-cleanup and depsgraph-and-evaluated-data. Unity is Y-up glTF (export_yup=True). Godot in this repo is Z-up glTF (export_yup=False). Unreal is centimeter scale on both exporters, and the RNA split is the contract: glTF has no global_scale / axis_forward / axis_up; FBX has those and has no export_yup.
• Snippets export_preset_unity.py, export_preset_godot.py, export_preset_unreal.py apply selected-mesh transforms via temp_override, pass use_selection, and treat Draco as opt-in (they do not duplicate gltf_draco_export.py).
• Rule use-correct-axis-rna-per-exporter plus a per-call extension of tests/check_import_export_rules.py (not a parallel harness). A mixed Unreal snippet that correctly calls both exporters still passes.
• Example export-preset-axis exports the same beacon under Unity and Godot, reads disk POSITION, re-imports, and asserts Unity stands while Godot lies along -Y. --same-axis exits 9.

Out of scope (untouched): templates/ai-asset-pipeline-template/, live-session agent bridge, LICENSE, issue #131, the 53 existing examples, generation-vendor integrations.

VERSION / CHANGELOG.md / CLAUDE **Version:** / ROADMAP **Current:** / plugin "version" are not hand-edited. Same convention as Phase 1 (#132); the release workflow owns those.

ROADMAP: cleanup sub-track marked Shipped (v0.54.0); engine export presets marked Shipped; template and agent bridge left unpinned. The Blender 5.2 LTS sweep line is already SHIPPED (PR smoke 5.2+4.5, weekly 5.1). There is no "July 2026" planned-sweep line to correct.

Axis RNA (docs + live binaries)

Official current docs (inspection-only as HTML; confirmed live on the three local binaries below):

• glTF export_yup: https://docs.blender.org/api/current/bpy.ops.export_scene.html#bpy.ops.export_scene.gltf
• FBX axis_forward / axis_up / global_scale: https://docs.blender.org/api/current/bpy.ops.export_scene.html#bpy.ops.export_scene.fbx
• 5.1 glTF: https://docs.blender.org/api/5.1/bpy.ops.export_scene.html#bpy.ops.export_scene.gltf
• 5.1 FBX: https://docs.blender.org/api/5.1/bpy.ops.export_scene.html#bpy.ops.export_scene.fbx

Live-run-proven RNA dump (bpy.ops.export_scene.{gltf,fbx}.get_rna_type().properties) on all three local builds. Identical on 4.5.11, 5.1.2, and 5.2.1:

Property	glTF	FBX
export_yup	yes	no
axis_forward	no	yes
axis_up	no	yes
global_scale	no	yes
apply_unit_scale	no	yes
export_apply	yes	no
use_mesh_modifiers	no	yes
use_selection	yes	yes
export_draco_mesh_compression_enable	yes	no

No version branch for the axis kwargs. The example guards by requiring every passed glTF kwarg to exist on operator RNA.

Task 3 canary (verbatim)

Fixture: .scratch/canary_axis_rna.py (gitignored, deleted after the run) with one export_scene.gltf(..., axis_forward="-Z", axis_up="Y") call and one export_scene.fbx(..., export_yup=True) call.

ERROR: .scratch/canary_axis_rna.py: export_scene.gltf call passes axis_forward or axis_up (FBX RNA; glTF uses export_yup)
ERROR: .scratch/canary_axis_rna.py: export_scene.fbx call passes export_yup (glTF RNA; FBX uses axis_forward / axis_up)

Harness exit 1. Clean scan (python tests/check_import_export_rules.py with no extra paths) then printed import/export anti-pattern checks passed. and exit 0, including the Unreal snippet that legally contains both export_yup (glTF) and axis_forward (FBX).

Example: live-run-proven

Probed and rendered on all three local binaries:

Binary	Reports
.scratch/blender-4.5.11-windows-x64/blender.exe	Blender 4.5.11 LTS (4db51e9d1e1e)
.scratch/blender-5.1.2-windows-x64/blender.exe	Blender 5.1.2 (ec6e62d40fa9)
.scratch/blender-5.2.1-windows-x64/blender.exe	Blender 5.2.1 LTS (9e2066aef7ef)

Check-only AABBs were byte-identical across the three:

source_aabb x=-0.4500..0.8400 y=-0.5700..0.4500 z=0.0000..2.1600
unity_reimport x=-0.4500..0.8400 y=-0.5700..0.4500 z=0.0000..2.1600
godot_reimport x=-0.4500..0.8400 y=-2.1600..0.0000 z=-0.5700..0.4500
unity_tip_err=0.000e+00 godot_tip_err=0.000e+00

--same-axis on 5.2.1: both reimports stand, stderr ERROR: orientations did not differ, exit 9.

Render path framing (all three): fill x=0.809 (band 0.70..0.90), margins >= 0.088, exit 0. No cross-version visual shim needed. Gallery hero/preview encoded with Pillow from the 5.2.1 PNG (1280x720 webp quality 85 / 1200x675); hero 8272 bytes, preview 7780 bytes.

Contact sheet: docs/gallery/contact-sheets/export-preset-axis-contact-sheet.webp vs pinned set armature-bend / damped-track-aim / bmesh-gear.

Image	Mean luma
armature-bend	71.08
damped-track-aim	43.89
bmesh-gear	77.71
export-preset-axis	46.84

Verdict: stage darkness in family with damped-track-aim (the dark member of the pinned set); warm wedge on the back wall; subject fill 0.809; olive + cyan beacon reads at thumbnail size; standing vs lying is the orientation contract. Not an asset-type example; no asset sheet.

Generated gallery HTML inspected: index card alt is the full teaches string (no dotted-path truncation); detail witnesses callout matches gallery.json.

Inspection-only

• Skill/snippet/rule prose and README wording.
• Official docs pages listed above (RNA also live-proven).
• Phase 3 template shape (not shipped).
• Godot engine is Y-up; this repo's Godot preset is Z-up glTF so the Unity/Godot pair is a measurable axis split. That naming is a product choice, documented in the skill.

Test plan

• python tests/check_import_export_rules.py exits 0
• Validate job (counts, manifest, harness)
• blender-smoke 5.2 LTS check-only export-preset-axis
• blender-smoke 4.5 LTS check-only export-preset-axis
• Drift-check
• Do not merge until both smoke legs are green