bench: add decode benchmark for extended bit depths

Measures decode throughput for 1024x1024 gray images at various bit
depths (3-24 bit). Requires pre-generated images in /tmp/bench-tiff/
(see stderr instructions for generation).

benches/decode.rs

@@ -0,0 +1,40 @@
+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();
+}