fix(relay): close F6 replay K_content nonce reuse via per-generation epoch-in-key

The recoverable replay key was stable per (hostContentSecret, sessionId) while the
agent-side sealer resets its deterministic-nonce seq to 0 on every restart/re-attach
→ two sealer generations sealed distinct plaintext under the SAME (key, nonce).

Add a required 'epoch' to ReplayKeyParams, fold it into the K_content HKDF salt
(sessionId U+001F epoch), mint a fresh epoch per createReplaySealer generation and
expose it, and thread it through ReplaySource so the browser re-derives the matching
key. Fresh epoch per generation ⇒ fresh key ⇒ seq=0 can never collide; recoverability
within a generation is preserved.

Touches relay-contracts/relay-e2e/agent/relay-web. Green: contracts 81, e2e 78,
agent 133, web 99; tsc clean. Regression proves same seq-0 nonce, different key.

relay-web/test/default-terminal.test.ts

@@ -52,10 +52,12 @@ describe('default (real-xterm) loaders behind the DI seam', () => {
 
   it('mountPreviewClient renders via the default read-only xterm loader', async () => {
     const secret = randomBytes(32)
-    const k = deriveContentKey({ hostContentSecret: secret, sessionId: 's', alg: 'aes-256-gcm' })
+    const epoch = 'gen-1' // FIX 3b / F6: seal + declare under the same generation.
+    const k = deriveContentKey({ hostContentSecret: secret, sessionId: 's', alg: 'aes-256-gcm', epoch })
     const replay: ReplaySource = {
       sessionId: 's',
       alg: 'aes-256-gcm',
+      epoch,
       frames: [encodeEnvelope(sealReplayFrame(k, 0n, new TextEncoder().encode('DEFAULT-XTERM')))],
     }
     const card = document.createElement('div')