fs: add macOS F_RDADVISE + micro-prefetch to accelerate SwiftCompile reads

[erneestoc]

Summary

Adds macOS `F_RDADVISE` + sequential-read hints + page-cache warming to the worker's read paths. This is the read-side complement to the write-side optimizations in #2338's APFS `clonefile(2)` work — and the macOS equivalent of the Linux `POSIX_FADV_SEQUENTIAL` hint that's already used on this codepath.

For a Bazel SwiftCompile p95 input shape (~466 MB, ~1,978 files), the read side dominates wall time. `F_RDADVISE` tells the macOS kernel "I'm about to read this file sequentially" — enables aggressive readahead and removes per-syscall stalls.

What changes

File	Change
`nativelink-util/src/fs.rs`	New `FileSlot::advise_sequential()` and `FileSlot::advise_willneed(offset, len)` helpers. macOS impl uses `libc::fcntl(F_RDADVISE)`. Linux impl uses existing `posix_fadvise` route. All other targets are `const fn` no-ops.
`nativelink-store/src/filesystem_store.rs`	`get_part` calls `advise_sequential` once on open; micro-prefetches the next 2 chunks inside the read loop.
`nativelink-util/src/store_trait.rs`	Same pair of hooks in `slow_update_store_with_file`.
`nativelink-worker/src/directory_cache.rs`	Fire-and-forget `warm_page_cache` after each successful hardlink cache hit (read path), via `tokio::spawn` with a bounded depth.

Implementation notes

• macOS's `F_RDADVISE` constant (`44`) and `radvisory` struct layout are not exposed by the `libc` crate. Both are declared locally with `#[repr(C)]` — verified against `<sys/fcntl.h>` on macOS 14+.
• cfg-gating is per-method so the three implementations of each name coexist cleanly: macOS gets the real syscall; Linux falls through to `posix_fadvise`; other targets are no-op `const fn`s.
• The page-cache warming on the DirectoryCache read path is fire-and-forget by design — it's a hint, not a correctness operation, and we don't want to block the materialize path waiting for it.

Tests

• `fs::advise_tests::macos_f_rdadvise_returns_success` — macOS-only, exercises the syscall path against a real `FileSlot` fd and asserts the kernel returned success.
• `fs::advise_tests::advise_helpers_are_callable_everywhere` — cross-platform sanity that the no-op compiles and returns successfully on Linux/Windows.

All 7 existing `fs` and `fs_util` tests pass.

Verification

• `cargo build -p nativelink-util -p nativelink-store -p nativelink-worker` clean
• `cargo test -p nativelink-util --lib` 7/7 pass; `cargo test -p nativelink-worker --lib` 3/3 pass
• `cargo clippy -p nativelink-util -p nativelink-store -p nativelink-worker --lib --tests -- -D warnings` clean for changed files (note: the 2 pre-existing clippy errors in `nativelink-store/src/r2_store.rs` from Replace bincode with wincode #2345 — `new_ret_no_self`, `single_match_else` — also fire on plain `origin/main`; unrelated to this PR)
• `cargo fmt --check` clean

Relationship to other work

• Complements macOS: APFS clonefile fast path + concurrency cap + zero-byte fix for Bazel input materialization #2338 (clonefile fast path on the write/materialize side).
• The DirectoryCache page-cache-warming runs on the READ path after a successful hardlink, in a detached `tokio::spawn` — does not contend with macOS: APFS clonefile fast path + concurrency cap + zero-byte fix for Bazel input materialization #2338's 64-cap on the materialize-path `hard_link` calls (different phase).
• Independent of worker: fix two silent output-corruption bugs (absolute symlinks + zero-digest files) #2346 (output-upload correctness) — that's the output-staging path; this is the input-reading path.

🤖 Generated with Claude Code

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