ffi: add rnp_uid_get_photo() for Photo ID read support (#13)

[ronaldtse]

Summary

Foundation for issue #1947 (Photo ID support). Adds an FFI function that parses the User Attribute packet body and returns just the image bytes plus the detected format, so callers don't have to implement the subpacket length decoding and image-header walk themselves.

What's in this PR

• include/rnp/rnp.h: new constants RNP_PHOTO_FORMAT_{UNKNOWN,JPEG,PNG} and new FFI function rnp_uid_get_photo(uid, &format, &data, &size) with thorough Doxygen.
• src/lib/userid.{hpp,cpp}: new helper rnp::parse_photo_attribute(uid_body, image, format) that walks the variable-length subpacket prefix, finds the first Image Attribute subpacket (type 1), and returns its image bytes plus detected format. The image header length byte counts itself (RFC 4880 §5.12.1 / RFC 9580 §5.13.1), so the remaining header bytes to skip are hdr_len - 1, not hdr_len. Format is detected from magic bytes (JPEG: FF D8 FF, PNG: 89 50 4E 47), not from the UserAttr header's format byte — real-world keys frequently leave that byte at 0 even when the image data is a valid JPEG.
• src/lib/rnp.cpp: FFI implementation. Returns RNP_ERROR_BAD_PARAMETERS if the UID is not a User Attribute packet, RNP_ERROR_BAD_STATE if the UserAttr body is malformed or contains no image subpacket.
• src/tests/ffi.cpp: extends the existing test_ffi_load_userattr to exercise the new API: asserts the photo UID returns JPEG bytes with the correct magic header, and that calling rnp_uid_get_photo() on a non-photo UID is rejected.

Why

The "gpg can, rnp can't" feature gap is most visible to end users — Thunderbird users with photo IDs on their keys see them silently dropped when their keyring is read by rnp. The UserAttr packet was already loaded as an opaque UID with type=RNP_USER_ATTR and the raw bytes retrievable via rnp_uid_get_data(), but parsing those bytes was the caller's job. This PR makes the common case (a JPEG photo) one FFI call.

Out of scope (follow-up PRs)

• rnp_key_add_photo() to attach a new photo to a key (needs serialiser work)
• CLI: --show-photo flag, [photo] indicator in --list-keys
• Negative tests with crafted malformed UserAttr bodies
• PNG photo round-trip test (would need a PNG fixture)

Test plan

• test_ffi_load_userattr (extended) passes: verifies JPEG magic, JPEG format constant, rejection of non-photo UID
• Manual verification against data/test_stream_key_load/ecc-25519-photo-pub.asc — image bytes form a valid JPEG (starts with FF D8 FF E0, ends with FF D9)
• No regressions in test_ffi_add_userid, test_ffi_key_userid_dump_has_no_special_chars, test_ffi_key_dump_edge_cases
• CI green

[codecov]

Codecov Report

❌ Patch coverage is 87.87879% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.47%. Comparing base (ce33cc6) to head (a9b409b).

Files with missing lines	Patch %	Lines
src/lib/rnp.cpp	77.77%	4 Missing ⚠️
src/lib/userid.cpp	91.66%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2430      +/-   ##
==========================================
+ Coverage   85.44%   85.47%   +0.02%     
==========================================
  Files         125      125              
  Lines       22962    23028      +66     
==========================================
+ Hits        19620    19683      +63     
- Misses       3342     3345       +3     

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[ni4]

LGTM!