fix(textures): enable compression extension before upload (fixes GL 1280)

[chiefcll]

Problem

Users on 1.4.0–1.4.3 hit WebGL: INVALID_ENUM (1280) and a blank/failed texture when loading .ktx (S3TC/ETC/ETC2) or .pvr (PVRTC) compressed textures. ASTC was unaffected.

Root cause

In WebGL, a compressed format enum is only valid in compressedTexImage2D after its owning extension has been enabled via getExtension — getExtension is the enable; there is no separate call.

uploadKTX/uploadPVR never call getExtension. They relied on the boot-time getWebGlExtensions() probe, which called getExtension on every compression extension as a side effect. #60 (1.4.0) correctly found the probe's result cache was dead and removed the whole function — but the getExtension calls themselves were load-bearing. uploadASTC survived only because it re-queries its own extension.

Result: the first KTX/PVR upload throws 1280. Not fixed by forceWebGL2 (#70) — WebGL2 still requires getExtension for these formats.

Fix

• Enable the owning extension at the point of use for all three upload paths (resolved by GL internal format), throwing a clear "...not supported by this device" error instead of leaking a silent 1280.
• Candidate extension-name lists hoisted to module constants — the resolver allocates nothing per upload. (getExtension is CPU-side + browser-cached and upload is not a per-frame path, so no runtime cache is warranted.)

Why it shipped: missing VRT coverage

tx-compression.ts existed but was a manual-only example (no automation/snapshot), so CI never exercised it. This PR promotes it to an automated VRT and commits a certified snapshot.

Also: two hung-test fixes

The VRT runner has no per-test timeout, so a never-resolving wait hangs the whole capture/compare run:

• tx-compression: an unsupported format on the running GPU (e.g. ETC1 under headless SwiftShader) surfaces as a failed texture that never fires loaded. Added a timeout backstop so the snapshot captures whatever the device produced.
• texture-free-reload: automation() ran after idle had already fired, so waitUntilIdle() waited forever. Force a final frame + short delay instead.

Tests

• New src/core/lib/textureCompression.test.ts — 8 deterministic unit tests asserting getExtension precedes compressedTexImage2D on every path, the WebKit-prefixed PVRTC fallback, and a clear throw (no upload) when unsupported.
• pnpm build, pnpm test, lint all clean.

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