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, 17 insertions, 1 deletions

diff --git a/src/util/asset_manager.h b/src/util/asset_manager.h
index 786a8db..f6ad244 100644
--- a/src/util/asset_manager.h
+++ b/src/util/asset_manager.h
@@ -16,10 +16,21 @@ enum class AssetType : uint8_t {
   PROC_GPU,
 };
 
+// Compression scheme applied to the packed bytes (only relevant for static
+// embedded assets; disk-load and procedural assets are always NONE).
+enum class AssetCompression : uint8_t {
+  NONE = 0,
+  // Order-0 rANS, model seeded from a corpus-wide histogram embedded in the
+  // generated assets blob (see GetAnsAsciiHistogram). Used for WGSL.
+  ANS_ASCII = 1,
+};
+
 struct AssetRecord {
   const uint8_t* data; // Pointer to asset data (static or dynamic)
-  size_t size;         // Size of the asset data
+  size_t size;         // Size of 'data' in bytes (compressed size if any)
   AssetType type;
+  AssetCompression compression;   // How 'data' is encoded; NONE = raw bytes
+  size_t uncompressed_size;       // Size after decompression (== size if NONE)
   const char* proc_func_name_str; // Name of procedural generation function
                                   // (string literal)
   const float* proc_params; // Parameters for procedural generation (static,
@@ -27,6 +38,11 @@ struct AssetRecord {
   int num_proc_params;      // Number of procedural parameters
 };
 
+// Initial-state histogram (256 entries) used to seed ANS_ASCII compression.
+// Defined in the generated assets blob; computed at pack time over the WGSL
+// corpus.
+const uint32_t* GetAnsAsciiHistogram();
+
 // Generic interface
 // Retrieves a pointer to the asset data.
 // - Static assets are guaranteed to be 16-byte aligned.