refactor(delib-1b): wasi path-based ops use Linux direct syscalls

[chaploud]

Summary

Second step of the W46 un-link-libc migration, building on #47.

• Add pfdMkdirAt, pfdUnlinkAt, pfdRenameAt, pfdReadlinkAt, pfdDup to src/platform.zig. Each uses std.os.linux.* direct syscalls on Linux, std.c.* on Mac/BSD, returns -1 on Windows (never reached — callers already branch to std.Io.Dir first).
• Update src/wasi.zig path-based ops: path_create_directory, path_remove_directory, path_unlink_file, path_rename, path_readlink, fd_renumber.
• Inline switch (comptime builtin.os.tag) for fd_filestat_set_times — Linux uses utimensat(fd, null, &times, 0) which is equivalent to futimens(fd, &times).

Behavior unchanged: existing cErrnoToWasi callers still work because the Linux branch mirrors the syscall errno into std.c._errno().* before returning.

Still not ready to flip link_libc = false

Remaining std.c.* references that will block un-linking:

• src/platform.zig:331,352 — std.c.getenv(...) (app-cache dir / env lookup)
• src/trace.zig:71 — std.c.write(stderr_fd, ...) (should be platform.pfdWrite)
• src/wasi.zig:525 — std.c._errno().* in cErrnoToWasi (need an alt errno source)
• src/wasi.zig path_filestat_set_times already uses std.c.utimensat on non-Linux (OK for Mac)

Those are W46 Phase 1c-1e, intentionally out of scope here.

Test plan

• zig build test (Mac aarch64) — all pass.
• zig build -Dtarget=x86_64-linux-gnu — clean.
• zig build -Dtarget=x86_64-windows-gnu — clean.
• bash test/realworld/run_compat.sh (Mac ReleaseSafe) — PASS: 50 / FAIL: 0.
• Full CI — Mac + Ubuntu + Windows green (46/46 real-world compat on Windows).

Tracks: W46 in .dev/checklist.md.