Persist engine mounts across seal/unseal cycles

- Add Registry.UnsealAll() that rediscovers mounted engines from the
  barrier on unseal, using stored metadata at engine/_mounts/ with a
  fallback discovery scan for pre-existing mounts (migration path)
- Registry.Mount() now persists mount metadata to the barrier;
  Registry.Unmount() cleans it up
- Call UnsealAll() from both REST and web unseal handlers
- Change Unmount() signature to accept context.Context
- Default CA key size changed from P-384 to P-521
- Add build-time version stamp via ldflags; display in dashboard status bar
- Make metacrypt target .PHONY so make devserver always rebuilds
- Redirect /pki to /dashboard when no CA engine is mounted

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/engine/ca/ca.go

@@ -1020,7 +1020,7 @@ func defaultCAConfig() *CAConfig {
 	return &CAConfig{
 		Organization: "Metacircular",
 		KeyAlgorithm: "ecdsa",
-		KeySize:      384,
+		KeySize:      521,
 		RootExpiry:   "87600h", // 10 years
 	}
 }