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/client/scanner.c

@@ -179,7 +179,7 @@ static bool entry_passes_selection(const FileListSet* file_list, const FilterRul
 static void scanner_capture_xattrs(const DirectoryScanner* scanner, File* file) {
   if (!scanner || !file || !(scanner->options.preserve_xattrs || scanner->options.preserve_acls))
     return;
-  file->xattrs = xattr_capture_path(file->path);
+  file->xattrs = xattr_capture_path(file->path, scanner->options.preserve_acls);
 }
 
 /* Apply --hard-links (-H) detection to one regular File.  On a sibling (a
@@ -1434,7 +1434,7 @@ static void scan_root_entry(const ScannerOptions* options, const FilterNode* roo
   }
   if ((options->preserve_xattrs || options->preserve_acls) &&
       !(file->link_group != 0 && !file->link_first))
-    file->xattrs = xattr_capture_path(file->path);
+    file->xattrs = xattr_capture_path(file->path, options->preserve_acls);
   if (!array_list_add(root_files, file)) {
     free(rel);
     file_destroy(file);