Add MEK rotation, per-engine DEKs, and v2 ciphertext format (audit #6, #22)

Implement a two-level key hierarchy: the MEK now wraps per-engine DEKs
stored in a new barrier_keys table, rather than encrypting all barrier
entries directly. A v2 ciphertext format (0x02) embeds the key ID so the
barrier can resolve which DEK to use on decryption. v1 ciphertext remains
supported for backward compatibility.

Key changes:
- crypto: EncryptV2/DecryptV2/ExtractKeyID for v2 ciphertext with key IDs
- barrier: key registry (CreateKey, RotateKey, ListKeys, MigrateToV2, ReWrapKeys)
- seal: RotateMEK re-wraps DEKs without re-encrypting data
- engine: Mount auto-creates per-engine DEK
- REST + gRPC: barrier/keys, barrier/rotate-mek, barrier/rotate-key, barrier/migrate
- proto: BarrierService (v1 + v2) with ListKeys, RotateMEK, RotateKey, Migrate
- db: migration v2 adds barrier_keys table

Also includes: security audit report, CSRF protection, engine design specs
(sshca, transit, user), path-bound AAD migration tool, policy engine
enhancements, and ARCHITECTURE.md updates.

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

web/templates/policy.html

@@ -38,6 +38,7 @@
                 <td>{{if .Actions}}{{range $i, $a := .Actions}}{{if $i}}, {{end}}{{$a}}{{end}}{{else}}<em>any</em>{{end}}</td>
                 <td>
                     <form method="post" action="/policy/delete" style="display:inline">
+                        {{csrfField}}
                         <input type="hidden" name="id" value="{{.ID}}">
                         <button type="submit" class="btn-danger btn-sm" onclick="return confirm('Delete rule {{.ID}}?')">Delete</button>
                     </form>
@@ -57,6 +58,7 @@
     <details>
         <summary>Add a new policy rule</summary>
         <form method="post" action="/policy">
+            {{csrfField}}
             <div class="form-row">
                 <div class="form-group">
                     <label for="rule_id">Rule ID <span class="required">*</span></label>