coverage: discover() returns unbakeable materials — coverage checker needs per-material capability classification

[gerchowl]

Problem

scripts/check_substrate_coverage.py (new in this branch) compares upstream discover() IDs against what's on HF. But discover() returns all materials an API advertises — including ones the baker can't produce texture tiers for. This creates permanent false-positive violations:

ambientcg (45 false positives):

• 28 materials offer only JPG downloads (adapter's _TIER_ATTRS matches only PNG)
• 17 materials have no downloadable files at all (empty/placeholder listings)

gpuopen (18 false positives):

• 18 procedural/parametric materials whose ZIPs contain only a .mtlx shader definition with zero texture PNGs
• _bake_mtlx() in bake.py:52-66 is a stub that raises NotImplementedError
• Materials: Aluminum, Brass, Bronze, Chrome, Cobalt, Copper, Gold, Iron, Nickel, Acryl Plastic, White Plastic, Frosted Glass, Glass, Dark Glass, Diamond, Red Wine, White Wine, Semitransparent Silicone

Proposed fix

Two-sided classification in discover() / the coverage checker:

Material capability	Expected texture tiers	Expected thumb	Example
textured (has PNG textures)	1k, 512, 256, 128, ktx2-*	yes	ambientcg/Bricks097, gpuopen/Wood_X
scalar-only (mtlx/parametric, no textures)	none	yes (shader-ball render)	gpuopen/Gold, gpuopen/Glass
unbakeable (no downloads / JPG-only)	none	no	ambientcg/Asphalt001 (JPG-only)

The coverage checker should:

1. Get (textured_ids, scalar_only_ids, unbakeable_ids) per source instead of one flat set
2. Expect texture tiers only for textured_ids
3. Expect thumb for textured_ids ∪ scalar_only_ids
4. Expect nothing for unbakeable_ids (or report them separately as "upstream has no PNG")

Where to implement

• _ids_for_source() in scripts/snapshot_upstream_catalog.py — return classified sets
• ambientcg: apply _filter_with_downloads() (already in sources/ambientcg.py:304) to split textured vs unbakeable
• gpuopen: check whether the material has texture packages vs mtlx-only to split textured vs scalar-only
• scripts/check_substrate_coverage.py — wanted_material_ids() returns classified sets, build_coverage_report() uses correct population per tier

Related

• bake(gpuopen): skip status=failed materials in thumb publish #428 — gpuopen status=failed materials leak into thumb publish (downstream symptom)
• Baking errors #285 — baking errors (broader)
• bake.py:52-66 — _bake_mtlx() stub (separate feature: MaterialX TextureBaker integration)

Optional follow-up

• JPG fallback for ambientcg's 28 JPG-only materials (update _TIER_ATTRS in sources/ambientcg.py)
• MaterialX TextureBaker implementation for gpuopen's 18 procedural materials