docs(#14646): record at both sites what the runtime pin stopped proving

Measured, one mutation and two pins: giving the shipped InMemoryRealtimeAdapter
a getChannelRoute() reddens service-realtime's no-channel-route pin (2 of 3
cases) and leaves the runtime producer pin green on all 4. So the shipped-
occupant claim is covered next door, not lost — and the runtime pin's stand-in
is equivalent to the real adapter only on the two reads a producer performs,
which was measured rather than assumed.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01ARYe3yQTQCUFm5qPYNgKaJ

Author: claude

.changeset/discovery-subscribable-channel-definition.md

@@ -17,4 +17,6 @@ What to read instead, per case:
 - deciding whether to open a subscription → `handlerReady === true && typeof route === 'string'`, i.e. `isSubscribableChannel(discovery.services.realtime)`, or the equivalent `capabilities.websockets.enabled`; poll or degrade otherwise;
 - asking whether the slot is occupied at all → `status` (`'unavailable'` = nothing registered; `'degraded'` = registered, reduced) — this is what `enabled` answered for `realtime` before.
 
+Testing note, recorded because it is a real limit rather than an implementation detail: the two producer pins drive a declared in-process-bus stand-in, not the shipped `InMemoryRealtimeAdapter` — `@objectstack/runtime` taking a source-level dependency on `@objectstack/service-realtime` for a test is refused by this repo's type-resolution ratchets. The claim about the shipped occupant is pinned against the real class in `@objectstack/service-realtime`'s own suite instead; a mutation giving that adapter a channel route reddens that pin and leaves the producer pins green, which is the division of labour stated at both sites.
+
 New in `@objectstack/spec`: `isSubscribableChannel()`, `readChannelRoute()`, `CHANNEL_SURFACE_SLOTS` (`@objectstack/spec/api`) and the optional `IRealtimeService.getChannelRoute()` — the producer half, by which an occupant that really serves a transport names the path a host mounted it at. Additive; no existing member changed shape. `@objectstack/service-realtime` deliberately does not implement it.

packages/runtime/src/discovery-realtime-channel.pin.test.ts

@@ -47,13 +47,28 @@ const PREFIX = '/api/v1';
  * `tsconfig.test.json` states it will not widen — 13 `TS6059` billed to a
  * ledger `service-realtime` cannot see, the same shape PR #12570 measured.
  *
- * Nothing is lost by declaring it here, because the claim about the SHIPPED
- * occupant is not this file's to make: it is pinned against the real class, in
- * the package that owns it, by
- * `packages/services/service-realtime/src/no-channel-route.pin.test.ts`. That
- * pin plus these compose to the stock-boot reading — this file pins that the
- * PRODUCER derives its answer from whatever the occupant names, and that one
- * pins what the shipped occupant names.
+ * ⚠️ What this file therefore STOPPED proving, stated because a pin that still
+ * passes while proving less is a real cost: the first case below no longer
+ * evaluates the SHIPPED adapter. Measured, one mutation, two pins —
+ * `InMemoryRealtimeAdapter` given a `getChannelRoute()` returning
+ * `/api/v1/realtime`: `no-channel-route.pin.test.ts` goes RED (2 of its 3
+ * cases), and THIS file stays GREEN on all 4. So if the shipped occupant ever
+ * starts naming a channel, the red arrives next door and never here.
+ *
+ * That is the whole of the loss, and it is covered rather than merely moved:
+ * the claim about the SHIPPED occupant is not this file's to make. It is pinned
+ * against the real class, in the package that owns it, by
+ * `packages/services/service-realtime/src/no-channel-route.pin.test.ts` — the
+ * pin the mutation above proves has teeth. That one pins WHAT THE SHIPPED
+ * OCCUPANT NAMES; this one pins that the PRODUCER derives its answer from
+ * whatever an occupant names. Together they compose to the stock-boot reading,
+ * and neither can go green by accident of the other.
+ *
+ * The stand-in is equivalent to the real adapter only on the two reads either
+ * producer performs on an occupant — `readChannelRoute` and
+ * `readServiceSelfInfo` — and that equivalence was measured (both `undefined`
+ * on both objects) rather than assumed. It is a statement about today, which is
+ * exactly why the shipped-occupant claim lives next door instead of here.
  */
 const inProcessBus: IRealtimeService = {
     publish: async () => {},