feat(astc): read upload capabilities from backend manifests

[apotema]

ASTC upload limits now come from the pinned backend's backend.manifest.zon, keyed by the resolved target name. The CLI no longer contains backend names or backend-version checks in src/astc/. The command reports whether the default came from a manifest or the conservative 4x4-only fallback.

The reader validates block lists and defaults strictly, supports local and cached remote package pins, and distinguishes an older manifest from a missing package. Malformed declarations stop the build rather than allowing a stale ASTC sibling to be consumed. Existing explicit/per-atlas/default precedence and stale-sibling handling remain. An explicit backend override cannot borrow the previous backend's metadata.

The companion bgfx manifest (labelle-toolkit/labelle-bgfx#157) declares 4x4/8x8 with an 8x8 default and PNG fallback on web. The assembler's runtime PNG fallback generation is unchanged; consuming the same fallback field there remains assembler #378 scope. Older backend manifests use 4x4 until their pins are updated. The converter is removed from the agnosticism guard's migration allowlist.

Validation:

• Zig 0.16.0 full CLI suite: 1,031 pass, 5 skip, zero failures.
• Tests distinguish manifest vs fallback even when both select 4x4, select different targets/version pins, and reject malformed/unsupported/duplicate blocks or a missing package.
• Real CLI comparison using four copied Flying Platform atlases: outputs are byte-identical to CLI v1.75.0 (characters/rooms/objects at 4x4, ship at 8x8). Switching the fixture from wasm to Android reuses all four outputs.
• No backend names or backend-version checks remain under src/astc/.

Release the companion backend declaration before upgrading projects that require the 8x8 default. This PR targets development and does not modify provider implementation files. The guard change removes one now-clean allowlist entry, so it may need reconciling with #422.

Closes #407.

Additional integration check: assembler 0.113.2 generated all four web atlas registrations through pickCompressedTexture(ASTC, PNG) with this manifest. The existing web PNG fallback is preserved.

All six CI jobs pass: Linux/macOS/Windows build and tests, Docker build, versions integration, and progress-feed end-to-end tests. CodeRabbit skips this development base under the repository configuration; the companion bgfx PR completed both bot reviews without findings.

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-27T01:40:49.769974Z	850f111	New commits

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