Add HEIC/HEIF test fixtures for upcoming heic-convert-based decoding

Reuses libheif's own LGPL/MIT-licensed with-alpha-512x512.heic sample
under both a .heic and .heif filename, verified to decode correctly
via heic-convert (the library the image format work will use). No
suitably-licensed HEVC/mif1-branded .heif-distinct sample exists that
is compatible with the pinned libheif-js version, and file-type detects
both extensions identically regardless of container brand, so the
duplicate content is sufficient to exercise the heif source-format
code path.

test/fixtures/HEIC_ATTRIBUTION.md

@@ -0,0 +1,16 @@
+# HEIC/HEIF fixture attribution
+
+`sample.heic` and `sample.heif` are byte-identical copies of
+`tests/data/with-alpha-512x512.heic` from the
+[strukturag/libheif](https://github.com/strukturag/libheif) repository
+(the reference HEIF/AVIF codec library), licensed under the project's
+LGPL-3.0/MIT terms (see that repo's `COPYING` file).
+
+Both filenames use the same content deliberately: `file-type` (the
+content-sniffing library this project uses) detects any file in the
+HEIC/HEIF family as `ext: 'heic'` regardless of the actual container
+brand, so the `.heic` vs `.heif` distinction in this project's code is
+purely about the declared source format taken from the upload's file
+extension, not about decoding a differently-branded container. Reusing
+one verified, cleanly-licensed sample under both extensions exercises
+that code path without requiring a second real-world sample.