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

benches/decode.rs

@@ -1,40 +0,0 @@
-extern crate criterion;
-extern crate tiff;
-
-use criterion::{black_box, Criterion, Throughput};
-use tiff::decoder::Decoder;
-
-fn decode_image(data: &[u8]) {
-    let image = std::io::Cursor::new(data);
-    let mut reader = Decoder::open(black_box(image)).unwrap();
-    reader.next_image().unwrap();
-    reader.read_image().unwrap();
-}
-
-fn main() {
-    let mut c = Criterion::default().configure_from_args();
-
-    // Check if large bench images exist
-    let bench_dir = std::path::Path::new("/tmp/bench-tiff");
-    if !bench_dir.is_dir() {
-        eprintln!("Skipping large image benchmarks: /tmp/bench-tiff/ not found");
-        eprintln!("Generate with: cc -O2 -o /tmp/gen_bench_tiff /tmp/gen_bench_tiff.c -ltiff -lm && /tmp/gen_bench_tiff");
-        return;
-    }
-
-    let mut group = c.benchmark_group("decode-1024x1024");
-    group.sample_size(30);
-
-    for depth in [3, 5, 7, 8, 10, 12, 14, 16, 24] {
-        let path = bench_dir.join(format!("bench-1c-{depth}b.tiff"));
-        if !path.exists() {
-            continue;
-        }
-        let data = std::fs::read(&path).unwrap();
-        group
-            .throughput(Throughput::Bytes(data.len() as u64))
-            .bench_function(format!("{depth}b"), |b| b.iter(|| decode_image(&data)));
-    }
-
-    group.finish();
-}