feat(assets): replace is_procedural/is_gpu_procedural bools with AssetType enum, add MP3 type

- Add AssetType enum {STATIC, PROC, PROC_GPU, MP3} to AssetRecord
- Add GetAssetType() API to asset_manager.h/cc
- asset_packer: parse 'MP3' compression keyword in assets.txt
- tracker: remove magic-byte is_mp3_asset(); use GetAssetType() instead
- assets.txt: NEVER_MP3 now uses 'MP3' compression type
- doc/ASSET_SYSTEM.md: rewritten to document new types and API

handoff(Gemini): AssetType enum landed; MP3 detection is now explicit via asset metadata.

1 files changed, 2 insertions, 11 deletions

diff --git a/src/audio/tracker.cc b/src/audio/tracker.cc
index 333a337..7ce96fc 100644
--- a/src/audio/tracker.cc
+++ b/src/audio/tracker.cc
@@ -44,15 +44,6 @@ static bool g_cache_initialized = false;
 // Forward declarations
 static int get_free_pool_slot();
 
-// Returns true if the asset blob looks like an MP3 file.
-static bool is_mp3_asset(const uint8_t* data, size_t size) {
-  if (!data || size < 3) return false;
-  // ID3v2 tag prefix
-  if (data[0] == 'I' && data[1] == 'D' && data[2] == '3') return true;
-  // Raw MP3 sync word: 0xFF followed by 0xE0-0xFF
-  if (size >= 2 && data[0] == 0xFF && (data[1] & 0xE0) == 0xE0) return true;
-  return false;
-}
 
 #if !defined(STRIP_ALL)
 // Decode an in-memory MP3 blob to a heap-allocated spectrogram (caller owns).
@@ -147,7 +138,7 @@ void tracker_init() {
         size_t size;
         const uint8_t* data = GetAsset(aid, &size);
 #if !defined(STRIP_ALL)
-        if (data && size > 0 && is_mp3_asset(data, size)) {
+        if (data && size > 0 && GetAssetType(aid) == AssetType::MP3) {
           int num_frames = 0;
           float* spec_data =
               convert_mp3_to_spectrogram(data, size, &num_frames);
@@ -165,7 +156,7 @@ void tracker_init() {
           }
         } else
 #else
-        FATAL_CHECK(data == nullptr || !is_mp3_asset(data, size),
+        FATAL_CHECK(data == nullptr || GetAssetType(aid) != AssetType::MP3,
                     "MP3 assets not supported in STRIP_ALL builds\n");
 #endif
         if (data && size >= sizeof(SpecHeader)) {