fix(xmp): handle near-limit XMP spill to Extended XMP, add fuzzer and fixture coverage

[DichenZhang1]

Summary

Follow-up polish on top of feat/preserve-xmp-metadata (09a3936) covering the near-limit Standard XMP spill boundary, duplicate #ifdef cleanup, fuzzer coverage, and real-world Apple gain-map fixture tests.

Changes

1. Near-Limit Standard XMP (~64.9 KB – 65,503 B) Spill to Extended XMP (lib/src/jpegr.cpp, lib/src/ultrahdr_api.cpp):

   • In JpegR::appendGainMap, when kWriteXmpMetadata (-DUHDR_WRITE_XMP=ON) is active and merging the Ultra HDR Container:Directory into a standard-sized user XMP packet (<= 65,503 bytes) pushes xmp_primary_str over kMaxStandardXmpPayload (65,503 bytes), revert xmp_primary_str to the unmerged user_xmp_str and spill it cleanly into Extended XMP chunks while writing only the Ultra HDR primary XMP + xmpNote:HasExtendedXMP GUID into the Standard XMP segment (preventing duplicate Container:Directory entries in Extended XMP).
   • Unify xmpAllowance in uhdr_encode (kJpegAppSegmentTotalMaxBytes + xmp_sz + num_chunks * 79) so near-limit standard XMP packets that spill into Extended XMP never overflow the output buffer.

2. Header Include Cleanup (lib/src/ultrahdr_api.cpp):

   • Remove the duplicate #ifdef UHDR_ENABLE_HEIF #include block.

3. Fuzzer Coverage (fuzzer/ultrahdr_dec_fuzzer.cpp, fuzzer/ultrahdr_enc_fuzzer.cpp):

   • Exercise uhdr_strip_gain_map (two-pass size query + strip) and uhdr_dec_get_xmp in both fuzzers without changing FuzzedDataProvider consumption order.

4. Unit Tests (tests/ultrahdr_api_test.cpp):

   • UltraHdrApiTest.StripGainMapOnAppleGainMapFixtures: Verifies uhdr_strip_gain_map on apple_gainmap_new.jpg and apple_gainmap_old.jpg (including DRI and RST0..RST7 markers in the primary scan).
   • UltraHdrApiTest.NearLimitStandardXmpSpillsCleanlyToExtendedXmp: Verifies a 65,200-byte user XMP packet spills cleanly to Extended XMP in -DUHDR_WRITE_XMP=ON mode and round-trips through uhdr_dec_get_xmp and uhdr_strip_gain_map.

Test Plan

• Verified ultrahdr_unit_test in both -DUHDR_WRITE_XMP=ON -DUHDR_WRITE_ISO=ON (26/26 UltraHdrApiTest, 34/34 JpegRTest, 17/17 JpegDecoderHelperTest) and -DUHDR_WRITE_XMP=OFF -DUHDR_WRITE_ISO=ON (21/21 UltraHdrApiTest).