fix(security): close every finding from the audit

Critical
- Any pubkey could publish a kind:30078 roles document and become root of a
  deployment, granting itself every capability and denying anyone. Roles are now
  accepted only from the configured root, compared against the constructor
  value rather than current authority state so a bad ingest cannot move the
  goalposts. Reproduced end to end before the fix; regression test added.
- Administrative state now fails closed without signature verification. Without
  it, event.pubkey is an unauthenticated claim, so an authorship check alone
  proves nothing — a relay can put the root's key on an event the root never
  wrote. trustUnsignedEvents opts in for local and test use.

High
- Report categories could evade moderation entirely: resolveThresholds used
  bracket truthiness, so "constructor" resolved to the Object constructor (all
  gates undefined) and "__proto__" to Object.prototype, both bypassing the
  default threshold. Own-property lookups and null-prototype tables throughout
  core — policy, evaluator, evidence, authority, presets.
- Live subscriptions bypassed signature verification entirely; an attacker only
  had to wait for the subscription rather than the load.
- The default administrative query asked relays for every kind:30078 and
  kind:30000 event in existence, with no author, identifier, or limit. Now
  filtered to governance identifiers with a bounded limit.

Medium
- Cached state was trusted as authority: a cache naming a different root is now
  rejected, so same-origin tampering cannot persist a takeover.
- Provider attribute changes silently stopped working after the first one, and
  concurrent starts leaked a transport. Generation-guarded.
- Unbounded growth in the report store, mute store, decision cache, and the
  transport's per-subscription dedupe set — all now capped.

Also restores the on-wire `d` identifier to <namespace>:governance:<scope>:v1.
The BitGate rename replaced "governance:" too broadly and silently changed the
event format; both encoder and parser changed together, so round-trip tests
never noticed. The identifier names the kind of document, not the library.

Adds docs/security.md and 21 regression tests reproducing each finding.

832 tests.

packages/runtime/src/commands.js

@@ -281,7 +281,7 @@ export class GovernanceCommands {
       kind: 30078,
       content: JSON.stringify(policy),
       tags: [
-        ["d", `${this.runtime.namespace}:bitgate:policy:v1`],
+        ["d", `${this.runtime.namespace}:governance:policy:v1`],
         ["v", "1"],
         ["client", "bitgate"],
         ["scope", "policy"],
@@ -307,7 +307,7 @@ export class GovernanceCommands {
     }
 
     const tags = [
-      ["d", `${this.runtime.namespace}:bitgate:community-sources:v1`],
+      ["d", `${this.runtime.namespace}:governance:community-sources:v1`],
       ["v", "1"],
       ["client", "bitgate"],
       ["scope", "community-sources"],