Improve directory cache filesystem performance

Rewrite hardlink_directory_tree, set_readonly_recursive, and
calculate_directory_size from async recursive to single spawn_blocking
with sync std::fs, eliminating ~5,550 async task transitions per large
tree. Add combined set_readonly_and_calculate_size function. Add macOS
clonefile(2) support in fs_util.

In directory_cache: parallel subtree clones with tokio::join!, skip
set_readonly on macOS (CoW makes it unnecessary), populate_fast_store_
unchecked for batch-checked blobs, page cache warming, zero-byte file
handling, format version bumps, EPERM fixes, subtree race fallback
with eviction recovery, and cfg-gate fixes.

Demote per-construction internal detail logs to debug! while keeping
cache HIT/MISS summaries and startup messages at info!.

Author: rejuvenile

.gitignore

@@ -25,4 +25,3 @@ nativelink.bazelrc
 *.log
 buck-out/
 .cargo/config.toml
-.claude/worktrees/