nothings/stb @ 31c1ad37456438565541f4919958214b6e762fb4

LOCAL PATCHES (re-apply after any re-vendor):
- stb_image.h, stbi__hdr_load flat/RLE-fallback loop: the upstream
  `stbi__getn(s, rgbe, 4);` ignores its return value, so a truncated Radiance
  .hdr leaves the per-pixel rgbe[] buffer uninitialized and leaks stack bytes
  into the decoded image (no error). Patched to check the return and fail with
  "corrupt HDR" (freeing hdr_data). Marked in-source with "[sceneio LOCAL PATCH]".
- stb_image.h, stbi__decode_jpeg_image marker loop: this pinned revision returns
  success when stbi__process_marker rejects a corrupt marker. That exposes
  partially uninitialized JPEG component buffers (a mutated DHT reproduced
  nondeterministic pixels across bytes/mmap and repeated calls). Current
  upstream returns failure. Patched `return 1` to `return 0`; marked in-source.
- stb_image_write.h, JPEG entropy writer: use an unsigned bit accumulator.
  Upstream's signed accumulator left-shifts negative values, which UBSan reports
  and C++ defines as undefined behavior. Marked in-source.
