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/extra-coverage.test.ts

@@ -207,7 +207,7 @@ describe('preview-client unavailable path', () => {
     const card = document.createElement('div')
     const client = mountPreviewClient(
       card,
-      { sessionId: 's', alg: 'aes-256-gcm', frames: [new Uint8Array([1])] },
+      { sessionId: 's', alg: 'aes-256-gcm', epoch: 'gen-1', frames: [new Uint8Array([1])] },
       new Uint8Array(32),
       { cols: 80, rows: 24 },
       {