## Summary
Add a 12-row reference table covering the full sRGB range with byte-exact 16-bit HLG and PQ signal values, plus three guard tests, locking down the `buildSrgbToHdrLut()` math.
## Why
`Chunk 9F` of `plans/hdr-followups.md`. The matrix-free fast path through `blitRgba8OverRgb48le` runs every DOM pixel through `buildSrgbToHdrLut()` (sRGB EOTF → linear → HDR OETF → 16-bit). Any drift in the EOTF/OETF math — constant changes, branch swaps, rounding-mode regressions — would silently corrupt every text / UI / overlay pixel composited onto an HDR frame.
Existing tests covered structural invariants (transparent passthrough, opaque overwrite, alpha blending, channel symmetry, HLG ≠ PQ) but no byte-exact reference values, so a uniform scale or constant tweak could pass everything.
## What changed
- 12-row reference table in `alphaBlit.test.ts` covering black, shadow, mid-grays, highlight, near-white, and white with exact 16-bit HLG and PQ signal values.
- Three guard tests:
- **Asymmetric R/G/B (HLG):** each channel hits the LUT independently.
- **Asymmetric R/G/B (PQ):** same, on the PQ path.
- **BT.2408 SDR-white invariant:** PQ caps sRGB 255 at 38055 (~203 nits), well below HLG's 65535. This is the load-bearing detail that makes PQ headroom work — locking the exact value prevents a future "fix" that would re-scale PQ to peak-at-SDR-white and clip every real HDR pixel.
Reference values mirror `buildSrgbToHdrLut()` exactly and were verified against the existing HLG mid-gray comment in the file.
## Test plan
- [x] All new tests pass against the current LUT.
- [x] Existing `alphaBlit.test.ts` invariants unchanged.
## Stack
Chunk 9F of `plans/hdr-followups.md`. Test-only change, independent of all other chunks.
