fix: prevent apply_commit from clobbering size/hash on generation mismatch

[XciD]

Summary

Fixes REVIEW.md finding 3 in #69 : apply_commit overwrote size and xet_hash unconditionally before checking clear_dirty_if(dirty_generation). If a concurrent writer advanced the generation, the in-progress file's size was clobbered with the stale flushed value.

Scenario:

1. Writer A flushes 10 MB file (captures gen=5)
2. Writer B extends file to 15 MB (gen=6)
3. Flush completes, apply_commit("hash", 10MB, gen=5) runs
4. Old code: sets size=10MB, then clear_dirty_if(5) returns false (gen is 6)
5. Applications see 10 MB until next flush, even though staging file is 15 MB

Fix: Move xet_hash, size, and pending_deletes.clear() inside the clear_dirty_if success branch.

One-line semantic change in src/virtual_fs/inode.rs, existing test updated.

[github-actions]

Benchmark Results

============================================================
  Benchmark — 50MB
------------------------------------------------------------
  Metric                                 FUSE          NFS
  ------------------------------ ------------ ------------
  Sequential read                    279.4 MB/s     248.8 MB/s
  Sequential re-read                1582.8 MB/s    2433.6 MB/s
  Range read (1MB@25MB)               32.6 ms         0.2 ms
  Random reads (100x4KB avg)          30.9 ms         0.0 ms
  Sequential write (FUSE)           1361.0 MB/s
  Close latency (CAS+Hub)            0.081 s
  Write end-to-end                   424.5 MB/s
  Dedup write                       1770.3 MB/s
  Dedup close latency                0.818 s
  Dedup end-to-end                    59.1 MB/s
============================================================
============================================================
  Benchmark — 200MB
------------------------------------------------------------
  Metric                                 FUSE          NFS
  ------------------------------ ------------ ------------
  Sequential read                   1030.6 MB/s     996.3 MB/s
  Sequential re-read                1624.2 MB/s    2473.1 MB/s
  Range read (1MB@25MB)               37.1 ms         0.2 ms
  Random reads (100x4KB avg)          32.6 ms         0.0 ms
  Sequential write (FUSE)           1568.4 MB/s
  Close latency (CAS+Hub)            0.104 s
  Write end-to-end                   864.9 MB/s
  Dedup write                       1653.6 MB/s
  Dedup close latency                0.088 s
  Dedup end-to-end                   958.8 MB/s
============================================================
============================================================
  Benchmark — 500MB
------------------------------------------------------------
  Metric                                 FUSE          NFS
  ------------------------------ ------------ ------------
  Sequential read                   1492.7 MB/s    1401.1 MB/s
  Sequential re-read                1787.0 MB/s    2451.5 MB/s
  Range read (1MB@25MB)               63.5 ms         0.2 ms
  Random reads (100x4KB avg)          32.5 ms         0.0 ms
  Sequential write (FUSE)           1450.4 MB/s
  Close latency (CAS+Hub)            0.464 s
  Write end-to-end                   618.5 MB/s
  Dedup write                       1409.8 MB/s
  Dedup close latency                0.118 s
  Dedup end-to-end                  1058.5 MB/s
============================================================
============================================================
  fio Benchmark Results
------------------------------------------------------------
  Job                        FUSE MB/s   NFS MB/s  FUSE IOPS   NFS IOPS
  ------------------------- ---------- ---------- ---------- ----------
  seq-read-100M                  369.0      454.5                      
  seq-reread-100M               2439.0       18.1                      
  rand-read-4k-100M                0.1        0.1         18         20
  seq-read-5x10M                 793.7      847.5                      
  rand-read-10x1M                  0.1        0.1         37         38
  Random Read Latency           FUSE avg      NFS avg
  ------------------------- ------------ ------------
  rand-read-4k-100M           54804.4 us   50735.1 us
  rand-read-10x1M             26970.8 us   26178.9 us
============================================================

[github-actions]

POSIX Compliance (pjdfstest)

============================================================
  pjdfstest POSIX Compliance Results
------------------------------------------------------------
  Files: 130/130 passed    Tests: 832 total (0 subtests failed)
  Result: PASS
------------------------------------------------------------
  Category               Passed    Total   Status
  -------------------- -------- -------- --------
  chflags                     5        5       OK
  chmod                       8        8       OK
  chown                       6        6       OK
  ftruncate                  13       13       OK
  granular                    5        5       OK
  mkdir                       9        9       OK
  open                       19       19       OK
  posix_fallocate             1        1       OK
  rename                     10       10       OK
  rmdir                      11       11       OK
  symlink                    10       10       OK
  truncate                   13       13       OK
  unlink                     11       11       OK
  utimensat                   9        9       OK
============================================================