fix(wasm): guard invoke_planar_ycbcr_u8 cwrap against stale WASM cache

cwrap for the new export was unconditional, so a stale CDN-cached WASM
binary without the export would crash _malloc during module init.
Guard with M['_invoke_decoder_planar_ycbcr_u8'] existence check and
fall back to invoke_planar_u8 (with CPU MCT) when unavailable.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: osamu620

web/shared/decoder_worker.mjs

@@ -111,7 +111,9 @@ async function init({ wasmBase = '/wasm/', threadCount: tc = 4, output = 'planar
     reset_decoder:     M.cwrap('reset_decoder_with_bytes','void',  ['number','number','number','number']),
     parse_j2c:         M.cwrap('parse_j2c_data',         'void',   ['number']),
     invoke_planar_u8:  M.cwrap('invoke_decoder_planar_u8','void',  ['number','number','number','number']),
-    invoke_planar_ycbcr_u8: M.cwrap('invoke_decoder_planar_ycbcr_u8','void', ['number','number','number','number']),
+    invoke_planar_ycbcr_u8: M['_invoke_decoder_planar_ycbcr_u8']
+      ? M.cwrap('invoke_decoder_planar_ycbcr_u8','void', ['number','number','number','number'])
+      : null,
     invoke_to_rgba:    M.cwrap('invoke_decoder_to_rgba', 'void',   ['number','number']),
     apply_bt601:       M.cwrap('apply_ycbcr_bt601_to_rgba','void', ['number','number']),
     apply_bt709:       M.cwrap('apply_ycbcr_bt709_to_rgba','void', ['number','number']),
@@ -281,8 +283,8 @@ function drainReady() {
       // Planar path — used by the WebGL2 renderer.  Three R8 textures get
       // uploaded on main; the fragment shader does the matrix.
       // For YCbCr codestreams, skip the CPU color transform and let the GPU
-      // shader handle YCbCr→RGB conversion — saves ~40% of decode time.
-      if (isYCbCr)
+      // shader handle YCbCr→RGB conversion — saves ~29% of decode time.
+      if (isYCbCr && F.invoke_planar_ycbcr_u8)
         F.invoke_planar_ycbcr_u8(decoder, yPtr, cbPtr, crPtr);
       else
         F.invoke_planar_u8(decoder, yPtr, cbPtr, crPtr);