feat(p7-output-fs): sparse hole preservation (-S), partial retention (-P), fake-super replay; block-size verified; crtimes/stderr -> Impossible/Divergence (Wave B)

- write_all_sparse: skips all-zero runs >= 4096 bytes via lseek(SEEK_CUR) and
  ftruncates the final size, wired into the atomic temp+rename and --inplace
  paths with no wire change (full image already in memory).
- --partial retention: on a save failure after the temp held data, rename the
  already-written temp to the destination path (best-effort; falls through to
  unlink; never retains when --partial is off) so --append/--append-verify can
  resume; tested by forcing futimens EINVAL with an out-of-range nsec.
- --block-size aliases --delta-block; verified config->delta_block_size is
  honored by the delta engine end-to-end (unit + integration tests).
- fake_super_restore_fd: parses and re-applies user.fastsync.stat fd-relative
  (fchown best-effort/non-root skipped, fchmod, futimens); a save under
  --fake-super now both records and re-applies.
- -N/--crtimes and --stderr=client promoted to a new 'Impossible/Divergence'
  status bucket (Summary: 136/2/4/3/2 = 147).
- cppcheck/clang-format clean; unit 37/37; integration 407 passed.

src/shared/xattr.h

@@ -86,4 +86,12 @@ bool xattr_apply_fd(int fd, const FileXattrList* list);
 void fake_super_store_fd(int fd, uint32_t uid, uint32_t gid, uint32_t mode, int64_t mtime_sec,
                          int64_t mtime_nsec);
 
+/* --fake-super replay: parse the FAKESUPER_XATTR record previously written on
+ * `fd` by fake_super_store_fd and re-apply uid/gid/mode/mtime fd-relative.
+ * Best-effort: absence of the xattr or a malformed record is a silent no-op
+ * that never fails the transfer, and fchown is applied only when permitted
+ * (a non-root EPERM is logged and skipped, matching FastSync's identity
+ * philosophy).  Returns true when the xattr was present and parsed. */
+bool fake_super_restore_fd(int fd);
+
 #endif