Remove regenerable editor cruft from version control (.blend1 backups, .spp autosaves, .assbin caches)

Summary

Several sample assets carry regenerable editor artifacts (Blender backups, Substance Painter autosaves, Assimp caches) that are committed to version control. These add no value to a consumer, are machine-generated, and are duplicated by git history itself. For a repo whose purpose is to model the canonical way to package SimReady assets, this is hygiene worth cleaning up.

This is separate from #12 (toaster dcc_source directory duplication), though that is the most extreme single instance.

Findings

1. .blend1 — Blender auto-backups (already gitignored, but still tracked)

Blender renames the previous save to *.blend1 on every save — it's a local undo backup, not an input or output.

• 15 files, ~78 MB, all tracked, every one paired with a real .blend sibling (purely redundant).
• .gitignore already lists .blend1 (line 74) — but these were committed before/around that rule, and .gitignore does not untrack already-committed files. They need an explicit git rm --cached.

2. *_autosave_*.spp — Substance Painter autosaves

• 1 file, 131 MB: obs_light_bulb_01/.../painter/light_bulb_autosave_0.spp — nearly as large as the real light_bulb.spp (132 MB). This single autosave is most of why the light-bulb asset is 308 MB.
• Not currently gitignored.

3. .assbin — Assimp binary cache

Regenerable import cache produced from the .fbx; not a source or deliverable.

• 4 files, ~52 MB (largest: obs_orange_a01/.../obs_orange_01_high.assbin at 37 MB).
• Not currently gitignored.

Context: where asset weight goes

Across sample_content/common_assets/props_general, ~83% of bytes are dcc_source/ authoring files (~2.76 GB) vs ~17% actual SimReady USD deliverables (~579 MB). Shipping the real .blend/.spp/.psd authoring sources is a deliberate "make assets re-authorable" decision and is out of scope here. This issue targets only the machine-generated backup/cache/autosave cruft above.

Suggested fix

1. Untrack the artifacts:
   • git rm --cached all *.blend1 (15 files)
   • git rm --cached **/light_bulb_autosave_0.spp (and any other *autosave*.spp)
   • git rm --cached all *.assbin
2. Extend .gitignore so they can't return:

   # Editor backups / autosaves / caches (regenerable, do not commit)
   *.blend1
   *autosave*.spp
   *.assbin
   

   (.blend1 is already present; the other two are new.)
3. Optional follow-up: git lfs prune after merge.

Estimated working-tree savings: ~260 MB (78 + 131 + 52), independent of the ~900 MB from #12.

Repro

find sample_content -type f -iname "*.blend1"      -exec du -ch {} + | tail -1
find sample_content -type f -iname "*autosave*.spp" -exec du -ch {} + | tail -1
find sample_content -type f -iname "*.assbin"       -exec du -ch {} + | tail -1

Start with .gitignore and the listed find commands to locate the tracked .blend1, autosave.spp, and .assbin files under sample_content. Untrack the identified artifacts, add ignore rules for the patterns not already covered, and verify that the unwanted files are removed from version control and cannot be re-added.