chore: replace decode_image fuzz target with comprehensive decoder exerciser

Replace decode_image with decode_comprehensive, which exercises
per-IFD via a probe_directory helper:
- Tag iteration and value parsing for every tag
- SubIFD, EXIF, and GPS directory traversal via read_directory
- Image metadata (dimensions, colortype, color_map, chunk info)
- Layout queries (image_buffer_layout, image_chunk_buffer_layout)
- Per-chunk reads at first and last indices
- Full image decode via read_image

Three IFD traversal strategies:
- Linear iteration via next_directory (tolerates invalid
  intermediate IFDs)
- Random-access via seek_to_directory (find_nth_ifd code path)
- Image-aware random-access via seek_to_image

Dictionary covers classic TIFF and BigTIFF byte order marks, and
common IFD tags in both LE and BE (image geometry, strips, tiles,
SubIFD, EXIF, GPS, ColorMap).

Strict resource limits (512 KB decode/intermediate, 64 KB IFD).
Bounded iteration (max 8 linear, max 4 seek).

README.md

@@ -55,4 +55,4 @@ This crate uses [cargo-fuzz](https://github.com/rust-fuzz/cargo-fuzz) in order t
 
 After installing it with `cargo install cargo-fuzz` on a nightly rustc, the
 fuzzing harness can be run with recommended settings using 
-`cargo fuzz run decode_image -snone -- -timeout=5`.
+`cargo fuzz run decode_comprehensive -snone -- -dict=fuzz/tiff.dict -timeout=5`.