v0.17.12: mapfile time-batched persistence — unblock NFS staging

Pre-0.17.12 every Mapfile::record() persisted the full mapfile via
tempfile-create + write + atomic-rename. On local LVM that's
microseconds; on NFS each rename is multiple RPCs through the
unraid user-share fuse layer, dragging a Black Mass UHD rip from
~11 MB/s on local to ~1.5 MB/s on NFS — the mapfile path alone burned
multiple seconds of wall time per real-world second of work.

Mapfile now batches the rename to once per second:

- record() always updates in-memory state and stats; only fires
  write_to_disk when last_flushed.elapsed() >= FLUSH_INTERVAL (1 s).
- New flush() API forces a persist; called by sweep_pipeline's
  consumer at end-of-sweep and by Disc::patch at end-of-patch,
  after the file's sync_all.
- Drop impl best-effort flushes so an early-return / unwind doesn't
  silently lose pending state.

Crash-safety changes from "lose at most one block" to "lose at most
1 s of recorded progress" — the ISO file's payload bytes are unaffected;
only the mapfile's authority over which sectors are already-good is at
risk, and a resume re-reads anything Pass 1 had already covered.

Measured on the BU40N test bed against Black Mass UHD inner zone:
- NFS staging: 1.5 MB/s → 16.48 MB/s (10.9× recovery)
- Local LVM staging: 11.09 MB/s → 11.83 MB/s (+6.7 % bonus)

Internal round_trip_load test now flushes before reading back from
disk. External patch / copy tests are unaffected: patch and
sweep_pipeline flush at completion before returning.

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "libfreemkv"
-version = "0.17.11"
+version = "0.17.12"
 edition = "2024"
 rust-version = "1.86"
 license = "AGPL-3.0-only"