fix(auth): address Copilot review round 5, and land the impl its tests need

Repairs origin as well as answering the review. 2d2c787e committed the
`absorbFileSecretsIntoKeyring` tests without the implementation they
import, so the branch has been failing `tsc` since; this commit brings the
source across.

Round 5's four findings (two inline, two suppressed):

- **The stale-lock takeover was still racy.** Round 4 made *stealing*
  liveness-aware but left the takeover as remove-then-recreate, which is
  not an election: two waiters can both judge the same lock stale, the
  first removes it and takes a fresh one, and the second then removes
  *that* — both inside the section, which is the lost update the takeover
  exists to prevent. Claiming is now an atomic `rename` to a
  token-unique name, so exactly one waiter can take a given stale lock and
  the losers get ENOENT. A failed claim also falls through to the deadline
  instead of `continue`ing, so a takeover that can never succeed times out
  rather than spinning forever on a swallowed error.

- **Every `stat` failure was read as "no file yet".** An existing file we
  cannot inspect (EACCES on the directory, a mode-less filesystem) then
  produced no warning at all, and the footer went on stating owner-only
  permissions having verified nothing. There is now a third state,
  `unknown`, carried end to end: `permissionsUnknown` on the descriptor, a
  caveat that says the permissions could not be checked, a `warn` tone, and
  its own branch in the footer sentence — which otherwise would have
  claimed "Owner-only permissions." by omission, the same bug one layer up.

- **mountinfo decoded only `\040`.** The kernel escapes tab, newline and
  backslash too (`\011`, `\012`, `\134`), so a mounted path containing any
  of them was compared in its escaped form, matched nothing, and read as
  the container's writable layer — `memory` selected and persistence
  silently lost on a box that had arranged for it. Decoded as octal in
  general now, since the rule is octal-in-general and a future addition
  would otherwise reintroduce exactly this.

- **File-backed secrets vanished once a keychain appeared.** Save secrets
  without libsecret, install libsecret, and the next run selects the
  keychain and stops seeing them: still on disk, read by nothing, and
  nothing looks broken. `absorbFileSecretsIntoKeyring` hands them over on
  the run that finds a keychain, under the rules the two existing plaintext
  migrations already use — keychain wins on conflict, the file is removed
  only on complete success, and a file that cannot be read is never
  deleted, because that is precisely the case where the values would be
  lost.

Tests: a two-waiter race over one stale lock asserting a single holder
throughout; `unknown` from `readSecretFilePermissions` through the
descriptor to the footer sentence and tone; the hand-off's happy path,
keychain-wins, write-failure, undecryptable-file, empty-file,
malformed-key, and explicitly-configured-keyring cases; and six more
rejected descriptor shapes, including each file-only field on a non-file
kind. Full `npm run ci` green — 5,732 tests under the coverage gate.

Signed-off-by: cliffhall <cliff@futurescale.com>

core/react/useInitialConfig.ts

@@ -111,6 +111,12 @@ function usableSecretStorage(value: unknown): SecretStorageInfo | undefined {
     if (v.looseMode !== undefined && typeof v.looseMode !== "number") {
       return undefined;
     }
+    if (
+      v.permissionsUnknown !== undefined &&
+      typeof v.permissionsUnknown !== "string"
+    ) {
+      return undefined;
+    }
   } else if (
     // File-only fields on a keychain or memory descriptor mean the payload
     // was not built by a backend we understand. Rejecting is cheap and the
@@ -118,7 +124,8 @@ function usableSecretStorage(value: unknown): SecretStorageInfo | undefined {
     v.path !== undefined ||
     v.plaintext !== undefined ||
     v.pendingEncryption !== undefined ||
-    v.looseMode !== undefined
+    v.looseMode !== undefined ||
+    v.permissionsUnknown !== undefined
   ) {
     return undefined;
   }