perf: use blake3 mmap_rayon for file hashing — zero heap allocation

- Replace std::fs::read() + update_rayon() with update_mmap_rayon()
  for file hashing, eliminating full-file heap allocation (500MB file
  no longer needs 500MB of RAM to hash)
- Enable blake3 'mmap' feature in Cargo.toml
- Lower hash_bytes rayon threshold from 10MB to 128KB
- Remove dead constants HASH_BLOCK_SIZE and RAYON_HASH_THRESHOLD

Cargo.lock

@@ -278,6 +278,7 @@ dependencies = [
  "cfg-if",
  "constant_time_eq",
  "cpufeatures",
+ "memmap2",
  "rayon-core",
 ]
 
@@ -1608,6 +1609,15 @@ version = "2.8.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "f8ca58f447f06ed17d5fc4043ce1b10dd205e060fb3ce5b979b8ed8e59ff3f79"
 
+[[package]]
+name = "memmap2"
+version = "0.9.10"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "714098028fe011992e1c3962653c96b2d578c4b4bce9036e15ff220319b1e0e3"
+dependencies = [
+ "libc",
+]
+
 [[package]]
 name = "mimalloc"
 version = "0.1.48"