Verify ACP relay events before prompt routing (#7010)

## What changed

Verify every Nostr EVENT received by buzz-acp immediately after
deserialization. Events with an invalid NIP-01 ID or Schnorr signature
are dropped before subscription routing, deduplication, replay-watermark
updates, membership handling, or either harness queue.

## Safety

Signature verification runs on the blocking pool so cryptographic work
does not block the relay task. A verification failure drops only that
event and keeps the connection available for subsequent valid traffic.
The existing observer-control verification remains as defense in depth.

Regression tests cover valid delivery; changed content, ID, signature,
pubkey, tags, and timestamp; a forged owner shutdown command with a
recomputed ID; forged membership notifications; and forged
observer-control events.

## Testing

cargo test -p buzz-acp

cargo clippy -p buzz-acp --all-targets -- -D warnings

just ci

---------

Signed-off-by: Jordan Mecom <jm@squareup.com>

Justfile

@@ -369,6 +369,9 @@ test-unit:
         cargo test -p buzz-auth --doc
         cargo nextest run -p buzz-voice --lib
         cargo nextest run -p buzz-cli
+        # buzz-acp owns the relay-to-agent trust boundary. Run its tests here so
+        # forged relay events cannot regain a path into agent routing unnoticed.
+        cargo nextest run -p buzz-acp
         # buzz-db migrator/lint tests: pure SQL-parsing unit tests (no infra).
         # They guard the embedded-migrator invariant (the complete checked-in
         # additive migration set; legacy cutover/backfill remains an operator