feat(protocol2): presence stream — online population + region transitions (Part B ph.2)

Overlay BridgePresence (new):
- presence.online sweep over online PlayerMobiles: total plus per-facet and
  per-region counts, emitted only when the population changes.
- region.enter real-time from EventSink.OnEnterRegion (player-filtered), the
  cheap location signal PLAN.md prefers over Movement.
- PresenceSweepSeconds (30s); wired into [bridge reload|sweepnow|status.

Sidecar:
- GET /online serves the latest presence.online snapshot from the event store
  (survives restart); population time series via /history?kind=presence.online.

Docs: INTEGRATION.md presence events + /online endpoint; PROTOCOL_2 ph.2 built.

Verified: sidecar cargo check clean; overlay compiles in the full ServUO
Scripts tree (0 errors, 0 warnings). Live run pending.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

link/INTEGRATION.md

@@ -280,6 +280,22 @@ In modern ServUO the "mayor" of a town is the **City Loyalty Governor**. The set
 
 Render the current board from `GET /governors` (§6) on connect, then keep it live with these events.
 
+#### Presence (Protocol 2.0)
+
+Who's online and where. A population snapshot is polled (`PresenceSweepSeconds`, default 30s) and emitted **only when it changes**; region transitions arrive in real time.
+
+| kind | fields | notes |
+|------|--------|-------|
+| `presence.online` | `count`, `byFacet` `{map: n}`, `byRegion` `{region: n}` | The current online population. Emitted when the count or any breakdown changes. `GET /online` gives the latest; `GET /history?kind=presence.online` the time series. |
+| `region.enter` | `from` (or null), `to` (or null), `map`, `who` (actor object) | A player crossed into a new named region. `from`/`to` are region names (`Wilderness` is unnamed). Cheap "who's where" feed. |
+
+```json
+{"kind":"presence.online","count":42,"byFacet":{"Felucca":12,"Trammel":30},
+ "byRegion":{"Britain":18,"Wilderness":9,"Despise":2},"t":1752489280000}
+{"kind":"region.enter","from":"Britain","to":"Despise","map":"Felucca",
+ "who":{"serial":"0x1A2B","name":"Darrow","acct":"whitlocktech","player":true},"t":...}
+```
+
 ---
 
 ## 5. REST — read queries
@@ -576,6 +592,16 @@ GET /governors
 
 Every city's latest governance snapshot — the live board, kept current by the `city.update` stream (§4). Empty if the shard does not run the City Loyalty system. Ordered by city.
 
+### Online population (Protocol 2.0)
+
+```
+GET /online
+→ {"kind":"presence.online","count":42,"byFacet":{"Felucca":12,"Trammel":30},
+   "byRegion":{"Britain":18,"Wilderness":9},"t":1752489280000}
+```
+
+The current online population — total plus per-facet and per-region breakdowns. The latest `presence.online` snapshot (from SQLite, so it survives a sidecar restart); keep it live with the `presence.online` stream (§4). `count: 0` with empty maps if the shard hasn't reported yet. For the population time series, `GET /history?kind=presence.online`.
+
 ---
 
 ## 7. Status codes

link/PROTOCOL_2.md

@@ -381,8 +381,8 @@ If the website mirrors rosters/links (it does — `store.record_link`), it must
 ## 13. Part B phasing
 
 1. ~~**Guilds + governors.**~~ **Built (2026-07-17), compiles clean both sides.** `BridgeSocial.cs` (guild sweep + `JoinGuild` → `guild.update`/`guild.remove`/`guild.join`) and `BridgeGovernance.cs` (city sweep → `city.update`, gated on `CityLoyaltySystem.Enabled`), `GuildSweepSeconds` (60s) / `CitySweepSeconds` (300s) config, both wired into `[bridge reload|sweepnow|status`. Sidecar `guilds`/`governors` board tables + `GET /guilds`, `/governors` served from the store (the §12.2 snapshot rule). Shared `BridgeJson.Actor` writer (serial/name/acct/webId/player). **Deviation from the §10 sketch:** the wire uses full-state `guild.update`/`city.update` upserts (website derives "created"/"governor changed" from the board) rather than discrete `guild.created`/`city.governor` events — this avoids a reconnect re-emit looking like a storm of creations, matching the proven `champ.update` model. *Live end-to-end run still pending.*
-2. **Presence.** Who's-online/population sweep + region presence (`OnEnterRegion`) → `GET /online`, population history in the store.
+2. ~~**Presence.**~~ **Built (2026-07-17), compiles clean both sides.** `BridgePresence.cs`: a `presence.online` sweep (total + per-facet + per-region, emitted on change) and real-time `region.enter` (`EventSink.OnEnterRegion`, player-filtered). `PresenceSweepSeconds` (30s), wired into `[bridge`. `GET /online` serves the latest snapshot from the event store (population series via `/history?kind=presence.online`). *Live run pending.*
-3. **Housing registry.** Extend the decay sweep to a full owner→houses list + houses-for-sale → `GET /houses`. (Selected from the §11 menu.)
+3. **Housing registry.** Extend the decay sweep to a full owner→houses list → `GET /houses`. (Selected from the §11 menu. Note: stock ServUO has no "for sale" flag on houses, so the registry is owner→houses; for-sale is dropped.)
 4. **Titles.** `char.profile` `titles` block (§10.3) — no new stream, folds into `BridgeProfile`.
 5. **Factions/VvV** — only after confirming which system the shard runs; stream just the enabled one.