fix(fs): tolerate FatFS VFS limits in atomic dir writes

[Love4yzp]

Problem

dir_write's atomic-overwrite path assumes two POSIX behaviors that ESP-IDF's FATFS VFS does not provide, so every atomic write through the fs module fails on ESP32 targets with a FatFS-backed root:

• O_EXCL opens fail with ENOENT on the FATFS VFS, so the temp-file create_new call never succeeds and the write errors out before any payload lands.
• FatFS f_rename refuses to overwrite an existing destination (FR_EXIST, surfaced as EEXIST) where POSIX rename replaces it, so rewriting an existing file fails even after the temp file landed and synced.

Fix

Both fallbacks arm only on the exact error kinds where the filesystem is known to diverge from POSIX:

• Temp-file creation falls back to create+truncate on ENOENT. The tmp dir is module-owned and swept, and the monotonic counter keeps the name unique, so losing O_EXCL costs nothing.
• The final rename falls back to remove+rename on EEXIST. The atomic-overwrite contract is not representable on FatFS, so the stale target is removed first.

Behavior on POSIX filesystems is unchanged — both fallback arms are unreachable there.

Verification

• cargo test -p pocket-fs — 12/12 pass on the host.
• Real hardware: Seeed reTerminal D1001 (ESP32-P4, ESP-IDF v6.1, FatFS on SD over the VFS) — data.fs writes and rewrites land and persist across reboots. Board-side receipts: pocketjs-d1001 verification doc.