feat(signing): default replay store + NewSignedHTTPClient preset

[bokelley]

Summary

Two ergonomic upgrades to bring `adcp-go` into parity with the cross-SDK signing ergonomics work:

• TypeScript (adcp-client#917, merged): defaults on `verifySignatureAsAuthenticator` + `createExpressVerifier`, default fetch on `buildAgentSigningFetch`, new `createAgentSignedFetch` preset.
• Python (adcp-client-python#272): default `replay_store`, `install_signing_event_hook` preset, migration guide.
• Go (this PR): default replay store, `NewSignedHTTPClient` preset.

Both backwards compatible — existing callers that pass explicit `Replay` or build the signing client by hand are unaffected.

1. `Middleware` defaults `Replay` to in-memory

`MiddlewareOptions.Replay` was a required field — passing nil silently disabled replay-check, the security regression AdCP verifier checklist step 13 exists to prevent. Now defaults to a fresh `NewMemoryReplayStore(0)` when nil, constructed once at wire-up so every request served by the middleware shares the same dedup state. Multi-replica deployments still pass an explicit shared store (Redis, etc.) to coordinate state across processes.

`Revocation` stays nil-on-omission — the existing wire-up warning ('RequiredFor is non-empty but Revocation is nil — verifier will not enforce key revocation') is the right behavior because revocation is operator-driven; defaulting it would silence a useful diagnostic.

2. `NewSignedHTTPClient(SignedHTTPClientOptions)` preset

Single-call buyer preset that bundles `NewSigner` + `RoundTripper` + a redirect-safe `*http.Client`. Disables `CheckRedirect` for you — `@target-uri` is part of the signature base, so a 3xx redirect would silently re-target the binding (the existing MIGRATION.md calls this out as a common production failure). Adapter authors who want this safety baked in get it without remembering to set `CheckRedirect` themselves:

```go
client, err := signing.NewSignedHTTPClient(signing.SignedHTTPClientOptions{
KeyID: "my-agent-2026",
PrivateKey: priv,
CoverContentDigest: true,
Timeout: 30 * time.Second,
})
```

Lower-level callers still reach for `NewSigner` + `RoundTripper` directly — those primitives are unchanged.

Test plan

• 6 new tests in `adcp/signing/client_test.go`:
  • `TestNewSignedHTTPClientRoundTripsAgainstVerifier` — preset → middleware round trip with Ed25519 key and seller-side verification.
  • `TestNewSignedHTTPClientRejectsRedirects` — confirms `CheckRedirect` is wired so a 3xx is surfaced to the caller, not followed.
  • `TestNewSignedHTTPClientRequiresKeyMaterial` — both empty options and missing fields error cleanly.
  • `TestNewSignedHTTPClientRespectsCustomInnerTransport` — inner transport composition order proven.
  • `TestMiddlewareDefaultsReplayStoreToInMemory` — middleware accepts signed requests with no `Replay` field set.
  • `TestMiddlewareDefaultReplayStoreRejectsActualReplay` — same (kid, nonce, body) replayed gets a 401 with `request_signature_replayed`.
• `go test ./...` clean across `adcp`, `adcp/idempotency`, `adcp/signing`, `adcp/webhook`.
• `go vet ./...` clean.
• `gofmt` clean.

Migration impact

None. Existing callers:

• `Middleware(MiddlewareOptions{Replay: myStore, ...})` → unchanged.
• `Middleware(MiddlewareOptions{...})` without `Replay` → previously had no replay protection, now has in-memory protection. Anyone affected was relying on a footgun.
• Manual `signer.RoundTripper(...)` integration → unchanged.

cc @benminer — this lands the Go half of the cross-SDK ergonomics work we discussed on adcp#3064. The Python sibling is at adcp-client-python#272.

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