webpdec.h: single-file WebP decoder

libwebp 1.6.0's decode + demux path (lossy VP8, lossless VP8L, VP8X
container, ALPH alpha, and the WebPDemux/WebPAnimDecoder animation API)
amalgamated into one stb-style header, built as portable C99 with all
SIMD, threads and host file I/O disabled.

amalgamate.py regenerates webpdec.h from a pinned, auto-cloned libwebp
checkout; tests/run.sh checks decode output bit-exactly against
reference libwebp.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitignore

@@ -0,0 +1,10 @@
+# Upstream libwebp checkout -- fetched on demand by amalgamate.py
+/libwebp/
+
+# Build artifacts
+*.o
+*.raw
+*.ppm
+__pycache__/
+
+# webpdec.h IS committed (it's the deliverable, regenerate with amalgamate.py)