feat(data): add 2k tier across all sources for v2026.05.1

[gerchowl]

Scope

`v2026.04.2` ships PNG `[1k, 512, 256, 128]` plus KTX2 `[ktx2-1k, ktx2-512]` (in flight). The original v2026.04.1 substrate (now sunset) had a 2k tier across all 3 textured sources — most upstreams natively serve 2k, and 2k is a meaningful step up for higher-fidelity GPU consumers. Add it back as part of the next release: v2026.05.1.

Plan

Branch v2026.05.1 from `v2026.04.2`'s tip commit (immutable inheritance, see #214 phase-7 chat — same pattern we'd have used for KTX2-only):

```bash
api.create_branch(branch="v2026.05.1", revision="<v2026.04.2 tip sha>")
```

Then on `mat-vis@v2026.05.1`:

1. Bake 2k natively from upstream for the 3 textured sources (ambientcg, polyhaven, gpuopen — all serve 2k natively per `source_tiers.SUPPORTED_TIERS`).
2. Optionally derive KTX2-2k from the new 2k tier (mirroring 1k/512 KTX2 coverage on v2026.04.2).
3. Update each catalog's `available_tiers` to include `"2k"` per record.
4. Manifest extends to `{src: {tiers: {1k, 512, 256, 128, 2k}}}` per source.

Cost estimate (rough)

2k textures are ~12 MiB/material × 7 channels × ~3175 materials = ~265 GiB total upload. With bytes-aware batching at 700 MiB/commit, that's ~380 commits — well above the 128/hr cap, so #227's 429 backoff helper will pace it. Expect ~2-4h wallclock for the matrix bake (sources serialize via `max-parallel: 1`).

Acceptance

• `mat-vis@v2026.05.1` branch exists, points at `v2026.04.2` tip + 2k bake commits.
• Manifest at `v2026.05.1` lists `2k` under each textured source's tiers.
• Catalog `available_tiers` includes `"2k"` for every textured-source record.
• Phase-5-style multi-language client smoke against `v2026.05.1` passes (the manifest-driven coverage from test(e2e): manifest-driven per-(source,tier) fetch coverage in all 4 client suites #248 will iterate the new tier automatically).
• Audit-orphans clean post-bake.
• Optional: KTX2-2k matrix derive after 2k PNG lands.

Pre-requisites

• v2026.04.2 fully shipped (we're a few commits away — KTX2 1k + 512 in flight).
• Spare HF commit budget when dispatched (avoid stacking with other prod work).

Out of scope for v2026.05.1

• 4k / 8k tiers (huge data; defer).
• physicallybased re-bake (still scalar — no native 2k tier exists).