refactor: simplify hash computation and remove unused benchmark

Address PR review feedback:

- Remove benches/decode.rs which referenced non-existent gen_bench_tiff.c
- Simplify compute_tiff_hash: replace 66-line match block with
  as_buffer().as_bytes() + ByteOrder::convert for LE normalization
- Use TIFF Type repr(u16) as hash discriminant instead of ad-hoc u8
- Rename all 74 seq test images to match updated hashes

Cargo.toml

@@ -54,9 +54,6 @@ harness = false
 name = "encode"
 harness = false
 
-[[bench]]
-name = "decode"
-harness = false
 
 [[test]]
 name = "decode_libtiffpic"