feat(admin): plugin write plane — admin.kick/ban/unban/broadcast

Phase 1 (plugin side) of docs/ADMIN_CONTROLS.md: a staff write plane so the
website can moderate the live shard.

- BridgeAdmin.cs: inbound admin.kick, admin.ban (timed + indefinite),
  admin.unban, admin.broadcast. Each requires an `actor`, refuses targets at
  or above AdminAccessFloor (default CoOwner — Owner-only shield), replies
  admin.ok/admin.error with the reqId echoed, and emits an admin.audit
  (origin=web) broadcast. Attribution is web:<actor> in the console log and
  the ban BanDealer tag. Kicking enumerates NetState.Instances so a
  character-select session is caught too.
- BridgeConfig/Bridge.cfg: AdminWriteEnabled (default OFF — opt-in),
  AdminAccessFloor, broadcast/reason length caps, ban duration clamp.
- tools/stub_sidecar_admin.ps1: live smoke-test harness; *.log gitignored.

Verified: compiles clean against ServUO (0 err/warn); live run on the seeded
shard confirms all four verbs, the audit stream, timed-ban fields, and the
Owner-floor refusal, with no exceptions.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0114TpmrNW4wNXsHq5CR72jQ

.gitignore

@@ -6,3 +6,4 @@ obj/
 *.dll
 *.exe
 *.pdb
+*.log