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.

agent/test/hostEndpoint.test.ts

@@ -107,6 +107,7 @@ describe('createE2ETransform (T15)', () => {
   function fakeReplay(): ReplaySealer & { calls: number } {
     const r = {
       calls: 0,
+      epoch: 'test-epoch',
       seal(_pt: Uint8Array) {
         r.calls += 1
         return { seq: 0n, nonce: new Uint8Array(), ciphertext: new Uint8Array([0xde]), tag: new Uint8Array() }