feat(verify): optional NIP-01 signature verification, on by default in the widget

Closes the gap the audit left open: administrative state fails closed without a
verifier, and BitGate shipped none.

@bitgate/verify is a separate package so core, nostr, and runtime keep no crypto
dependency and can be audited without one. The widget bundles it, so the
drop-in path is verified with no extra step; verify="off" disables it, sets
data-unverified on the element, and emits bitgate:unverified — meant for local
development, and meant to be visible.

Verification recomputes the event id before checking the signature. Checking
only the signature would let a forged event pass by reusing a genuine signature
over an unrelated id, which is the failure mode that matters here.

createVerifier also rejects events dated far in the future. A far-future
created_at wins replaceable-event selection indefinitely and pins stale state in
place; the signature is perfectly valid, so only a clock check catches it.

Found while testing: #adminIngestAllowed checked that a verifier *exists*, not
that this event was verified — so the public ingestEvent path, which
"bring your own relays" integrations use, bypassed verification entirely.
Verification now runs inside ingestEvent, with an already-verified flag for the
loaders. An async verifier cannot be awaited there and is treated as
unverified; ingestVerified() awaits properly.

Conformance validated against 120 live events from a public relay across kinds
0, 1, 3 and 10002 — 59 with non-ASCII content and 71 containing quotes,
newlines or tabs, which is where a serialization mismatch would surface. All
120 verified, zero id mismatches.

Bundle grows 72K -> 120K. 855 tests; audit clean.

README.md

@@ -46,6 +46,7 @@ equivalent to following the operator's seed list.
 | `@bitgate/core` | Pure evaluation. No I/O, no browser globals, no dependencies. |
 | `@bitgate/nostr` | Codecs: canonical v1, NIP-56, NIP-51, legacy lists, bech32. |
 | `@bitgate/runtime` | Stores, transport orchestration, commands. |
+| `@bitgate/verify` | Optional NIP-01 signature verification, so the core stays dependency-free. |
 | `@bitgate/testing` | Conformance harness. |
 | `@bitgate/widget` | Drop-in custom elements: viewer controls and a moderator console. |
 | `@bitgate/bitvid-compat` | Reference application profile for the characterization corpus. |