fix(a7-auth): publish dummy-key sidecar atomically and harden reads

Address review findings on the persistent dummy-key sidecar:

- Publish atomically: write a private same-directory temp file
  (<store>.dummykey.tmp.<pid>, 0600), fsync, then link(2) into place;
  fsync the containing directory and drop the temp name. A concurrent
  starter can no longer observe a zero/partial sidecar and fail closed.
  On EEXIST adopt the winner's sidecar; otherwise warn and use a
  transient ephemeral key.
- Harden the read path (initial and EEXIST-adopt) with
  O_RDONLY|O_NOFOLLOW|O_NONBLOCK|O_CLOEXEC: reject planted symlinks
  (ELOOP fails closed) and never block on a planted FIFO.
- Require the exact owner-only mode (st_mode & 07777) == 0600 and make
  the rejection message truthful.
- Report a clear "short write" instead of a stale strerror(errno) when
  write() returns 0.
- Document the artifact and its creation-failure caveat (FIFO store
  path, read-only filesystem, missing directory) in README.md and
  RSYNC_COMPAT.md.
- Tests: known-key sidecar adoption (dummy salt KAT + reload), symlink
  rejection, and the exact-0600 rule (0400 now rejected).

README.md

@@ -526,7 +526,11 @@ redirect that output to an owner-only (mode 0600) file, and note that legacy
 (mode 0600) `<store>.dummykey` sidecar next to the store: it holds the store-wide
 dummy key, is auto-created on first load, and must be preserved across daemon
 restarts so the dummy challenge for an unknown user stays stable (the key is
-never regenerated while the sidecar exists). One residual is accepted: the store
+never regenerated while the sidecar exists). If the sidecar cannot be created
+(process-substitution/FIFO store path such as `/dev/fd/N`, a read-only
+filesystem, or a missing directory), the daemon logs a warning and uses a
+transient key, so the cross-restart guarantee does not hold for those
+deployments. One residual is accepted: the store
 iteration count is observable pre-auth by design, since the miss path must match
 a hit.