ans: order-0 rANS coder + WGSL asset compression

Adds src/util/ans.{h,cc}, a per-chunk-adaptive order-0 rANS entropy
coder. Decoder is always built; encoder is gated on ANS_ENABLE_ENCODER
(tools only). Both sides take an optional 256-entry initial_counts
table to seed the adaptive model.

The per-chunk initial state is (1 << kBits). Higher initial states
(e.g. with a signature packed into the upper bits) force a renorm-emit
at iter 0 that the decoder never consumes, corrupting multi-chunk
streams once stats become skewed.

Asset pipeline:
- AssetRecord gains 'compression' and 'uncompressed_size' fields.
- asset_packer scans every WGSL file to build a corpus-wide byte
  histogram, then ANS-encodes each shader using that histogram as the
  seed. Histogram and accessor are emitted alongside the asset table.
  Round-trip verification runs at pack time for every compressed
  asset; failures fall back to uncompressed storage.
- asset_manager decompresses on first GetAsset(), caches the
  heap-allocated buffer, and DropAsset / ReloadAssetsFromFile free it
  along with the procedural cache.
- Disk-load (dev) builds are unchanged: WGSL paths stay as filenames.

Tests:
- src/tests/util/test_ans.cc: roundtrip variants (empty, single byte,
  single-symbol run, all-zeros, random uniform/skewed, repeated ASCII),
  seeded-vs-uniform compression, rejection of mismatched counts /
  corruption / truncation, PeekUncompressedSize.
- 37/37 dev, 36/36 STRIP_ALL.

Compression observed: WGSL shaders shrink to ~0.62-0.71x in the main
workspace (81 of 105 assets qualify).

Docs:
- doc/ANS.md (new): algorithm, bitstream, API, asset pipeline
  integration, compression numbers, limitations, tests.
- doc/ASSET_SYSTEM.md: new Compression section + updated technical
  guarantees for compressed assets.
- doc/COMPLETED.md: May 2026 entry.
- PROJECT_CONTEXT.md: Build status line mentions WGSL ANS compression.
- CLAUDE.md, GEMINI.md: tier-3 build doc list includes ANS.md.

1 files changed, 4 insertions, 0 deletions

diff --git a/doc/COMPLETED.md b/doc/COMPLETED.md
index 233373e..bf4c3ba 100644
--- a/doc/COMPLETED.md
+++ b/doc/COMPLETED.md
@@ -34,6 +34,10 @@ Completed task archive. See `doc/archive/` for detailed historical documents.
 
 ---
 
+## May 2026
+
+- [x] **ANS shader compression (2026-05-14)** — Order-0 rANS coder in `src/util/ans.{h,cc}` (decoder always built; encoder gated on `ANS_ENABLE_ENCODER`). `asset_packer` derives a corpus-wide byte histogram from every WGSL file, ANS-encodes each shader with that seed, and round-trip-verifies at pack time. `AssetRecord` gains `compression` + `uncompressed_size`; `asset_manager` decompresses lazily on first `GetAsset()` and frees in `DropAsset`/`ReloadAssetsFromFile`. WGSL assets shrink to ~0.62–0.71× in `workspaces/main` (81/105). See `doc/ANS.md`. Tests: 37/37 dev, 36/36 STRIP_ALL.
+
 ## March 2026 (continued)
 
 - [x] **FFT twiddle factor fix** — `fft_radix2` computes `wr/wi` directly per k via `cosf/sinf(angle*k)`. Tests A–E added to `test_fft.cc`. Tolerance reverted to 5e-3.