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/file.h

@@ -113,12 +113,14 @@ bool file_to_disk_secure_no_replace(const char* path, const void* data,
                                     const char* temp_dir);
 /* Receiver write-path variant that also applies per-file xattrs (-X/-A) and the
  * --fake-super stat xattr fd-relative before the final rename.  `update` /
- * `no_replace` / `use_fsync` mirror the plain wrappers above. */
+ * `no_replace` / `use_fsync` mirror the plain wrappers above; `keep_partial`
+ * enables --partial best-effort retention of a failed write's temp. */
 bool file_to_disk_secure_attrs(const char* path, const void* data, unsigned long long data_size,
                                bool inplace, bool sparse, bool preallocate,
                                const FileMetadata* metadata, bool preserve_executability,
                                bool update, bool no_replace, bool use_fsync,
-                               const FileXattrList* xattrs, bool fake_super, const char* temp_dir);
+                               const FileXattrList* xattrs, bool fake_super, bool keep_partial,
+                               const char* temp_dir);
 /* Atomic --link-dest install: replace `path` with a hard link to `basis_path`
    (via a temp name + rename); fall back to a byte-identical local copy from
    `data` when the link is impossible (EXDEV/EPERM/unsupported filesystem).