fix(receiver): close re-review findings — dry-run basis oracle, ACL capture, fsync reopen

Follow-up to a237043 addressing three security/correctness re-review findings.

(1) MEDIUM: a server-contacting --dry-run with --compare-dest/--copy-dest/
    --link-dest still read and hashed the basis file and compared it with the
    client-supplied digest, a 1-bit content oracle. basis_match_find() gains a
    hash_content parameter; the dry-run shortcut passes false and returns no
    match without touching basis bytes, so an otherwise-matching entry is
    reported as would-transfer. The real (non-dry-run) path is unchanged.

(2) LOW: xattr_capture_path() hardcoded preserve_acls=true, so the receiver's
    hard-link copy fallback re-applied system.posix_acl_* even when -A was not
    negotiated. The function now takes preserve_acls and members.* is
    unaffected; scanner and receiver callers thread the negotiated flag.

(3) INFO: the --fsync --link-dest temp reopen now uses O_NONBLOCK and treats
    a raced-in FIFO's ENXIO as a benign fsync-skip instead of blocking.

Tests: dry-run + basis unit test (asserts would-transfer, no content read) and
integration test; xattr capture ACL-filter test. Verified strict build, ASan,
clang-format, cppcheck, and the CI integration subset.

src/shared/xattr.h

@@ -65,10 +65,13 @@ bool xattr_list_append(FileXattrList* list, const char* name, const void* value,
  * Used for both capture and receiver-side validation. */
 bool xattr_name_appliable(const char* name, bool preserve_acls);
 
-/* Sender: read the whitelisted xattrs of `path` into a new list.  Returns NULL
- * when the path has no appliable xattrs (or the filesystem has no xattr
- * support); an empty-but-valid list is never returned distinct from NULL. */
-FileXattrList* xattr_capture_path(const char* path);
+/* Sender: read the whitelisted xattrs of `path` into a new list.  The POSIX ACL
+ * names are captured only when `preserve_acls` (--acls/-A) is set, so a plain
+ * -X run never carries an ACL it was not asked to preserve; `user.*` is
+ * unaffected.  Returns NULL when the path has no appliable xattrs (or the
+ * filesystem has no xattr support); an empty-but-valid list is never returned
+ * distinct from NULL. */
+FileXattrList* xattr_capture_path(const char* path, bool preserve_acls);
 
 /* Wire: bounded serialization.  xattr_send returns false on write failure; an
  * empty/NULL list transmits a zero-count block.  xattr_receive returns NULL and