Sniff stream signature to preserve sub-format detection

[davidbyttow]

Summary

Follow-up to #539, fixing the issue Macroscope flagged there: the streaming load derived the image type from the loader nickname alone, so AVIF (via heifload_source) reported ImageTypeHEIF, and BMP/PSD (via magickload_source) reported ImageTypeMagick with the original format lost. The buffer path detects these from the file signature — and ExportNative on a streamed AVIF would have picked the HEIC encoder.

Fix: sniff the first 12 bytes before loading (vips_source_sniff buffers and rewinds, so nothing is consumed, and it works for every source kind at that point) and refine the ambiguous HEIF/MAGICK loader types through the existing DetermineImageType, mirroring vipsLoadFromBuffer's current/original format split exactly.

Tests

• New TestLoadImageFromReader_SubFormatDetection: for avif-8bit.avif, bmp.bmp, and psd.example.psd, streaming Format()/OriginalFormat() must equal the buffer path's. BMP/PSD verified locally; AVIF exercises on CI (local HEIF module is broken — missing libx265).
• Full streaming suite locally: identical results to the pre-change baseline (only the known environmental HEIC failures).

🤖 Generated with Claude Code

Note

Sniff stream signature to preserve sub-format detection in LoadImageFromReader

• Adds a source_sniff_header C helper in stream.c that reads up to N bytes from a VipsSourceCustom without consuming the stream, using vips_source_sniff and memcpy.
• LoadImageFromReader in stream.go now sniffs up to 12 bytes before loading, then uses DetermineImageType to refine OriginalFormat when the loader reports HEIF or Magick (e.g. resolving to AVIF, BMP, or PSD).
• Format is also set to the sniffed sub-format when the detected type does not require Magick, matching the behavior of LoadImageFromBuffer.
• A new test in stream_test.go asserts that Format and OriginalFormat agree between the buffer and reader load paths for AVIF, BMP, and PSD inputs.

^{Macroscope summarized 14a9aee.}