release-cut: v0.6.0 full matrix bake + promote dev → main

[gerchowl]

Tracking issue for the v0.6.0 release cut. Sequence locked in after
#178 landed:

• 1. Full staging bake on anvil-dev → `gerchowl/mat-vis-tst@v0.0.4-staging`
  • All 4 sources × all tiers where applicable
  • Includes KTX2 derives (1k, 2k)
  • Wall-clock: 4-8 h (upstream rate limits)
• 2. Verification against staging:
  • `uv run pytest tests/` (320)
  • `HF_INTEGRATION=1 uv run pytest tests/test_mat_vis_block_roundtrip.py` (5)
  • `dagger -m .dagger call validate-release` (rowmap ↔ tar, catalog completeness)
  • Client smoke: `MatVisClient(tag="v0.0.4-staging").fetch_texture(...)` round-trip on one material per source
• 3. Release gate: `git checkout main && git merge --ff-only dev && git push`
• 4. Full prod bake on anvil-dev from `main` checkout → `gerchowl/mat-vis@v2026.05.1` (pick calver)
  • Same commands as step 1 but with `--repo-id=gerchowl/mat-vis --allow-prod=true --release-tag=v2026.05.1`
• 5. Tag the release: `git tag v2026.05.1 && git push origin v2026.05.1`
• 6. Client release: bump `clients/python/pyproject.toml` to 0.6.0, tag, publish via `pypi.yml`

Why not a promotion script

Deterministic bakes (from #178) + atomic HF commits mean a bake from
`main` produces the same catalog shape as a bake from the same
commit against staging. The delta is whatever upstream changed between
the two runs, which IS the point of the calver release. No
artifact-copy tooling needed.

Rollback

Each bake is its own atomic HF commit on its own tier/source path.
If a single (source, tier) errors in step 4, retry just that one —
the rest of the release revision is unaffected.

If the entire calver needs rollback after tag: `git push origin
:v2026.05.1` to untag, `hf api datasets/gerchowl/mat-vis/revision/v2026.05.1 -X DELETE`
to drop the HF revision pointer (revisions are branches, so this
just removes the label — the commits stay reachable via their SHAs).

What's NOT needed

• Promotion script (dropped — deterministic rebake is cleaner).
• Staging→prod artifact copier.
• New CI workflows (the `bake.yml`/`derive.yml` dispatch paths already exist as public-contributor fallbacks; for this release we run on anvil-dev for speed).

Refs #178, ADRs 0007/0008/0009/0010/0011.