Phase 2a: FAT engine mutations + O_TRUNC (fix the overwrite corruption)

The FAT engine could create, read, write, and grow files, but never free clusters
or make directories — so overwriting a shorter file left a stale tail (a real bug:
corrupt boot-log re-flushes, and later corrupt compiler cache/.o files). This adds
the mutation half of the engine, with the corruption fix wired all the way through.

Engine (system/services/fat/engine.zig), all host-tested:
- freeChain: return a cluster chain to the pool (bounded against a corrupt cycle) —
  the shared primitive under truncate and remove.
- truncate: free the chain and zero the entry's size/first-cluster (O_TRUNC).
- createDirectory (mkdir): allocate + initialise a cluster with "." and ".." and add
  the directory entry to the parent.
- removeFile (unlink): free the chain and mark the 8.3 entry plus any preceding
  long-name entries deleted, so a reused slot can't inherit an orphaned long name.
  Refuses directories.
- createFile now shares a common addEntry helper with createDirectory.

O_TRUNC wired end to end: a truncate open-flag (vfs protocol) that the router already
forwards; runtime.fs.OpenOptions.truncate; and fat's handleOpen calls engine.truncate
on an existing file. The boot-log flush (log-flush + init) now opens with truncate, so
a shorter log on a later boot of the same stick leaves no stale tail — closing the
caveat from the boot-log work.

mkdir/unlink are engine-complete and host-tested but not yet exposed as VFS
operations / runtime.fs methods (they are new path-based ops needing router cases);
rename and the richer stat (mtime/mode, blocked on wall-clock) remain. See
docs/zig-self-hosting.md (Phase 2).

Verified: zig build, zig build test (8 engine host tests, incl. truncate, the
overwrite-no-stale-tail regression, remove, and mkdir), zig build check-fat-image, and
a sequential QEMU sweep — fat-mount, log-flush (DANOS.LOG read back at 11804 bytes,
clean, with the truncate-based final flush), vfs, vfs-client-death, usb-storage,
orderly-shutdown, initial-ramdisk, smoke — all green.

system/services/vfs/protocol.zig

@@ -82,11 +82,15 @@ pub const reply_size: usize = @sizeOf(Reply);
 /// Largest inline payload that still fits one IPC message alongside a header.
 pub const maximum_payload: usize = message_maximum - request_size;
 
-/// Open flags (danos-native; the POSIX layer maps `O_CREAT` onto `create`).
+/// Open flags (danos-native; `runtime.fs.OpenOptions` maps its booleans onto these).
 pub const create: u32 = 1;
 /// Open a directory (for readdir) rather than a file. A mounted backend uses
 /// this to open a directory node; the flat ramfs ignores it.
 pub const directory: u32 = 2;
+/// Truncate the file to zero length on open (O_TRUNC): replace its contents rather
+/// than overwriting in place, so a shorter new file leaves no stale tail. A mounted
+/// backend frees the old cluster chain; the flat ramfs ignores it.
+pub const truncate: u32 = 4;
 
 test "protocol struct sizes and node kinds" {
     const std = @import("std");