refactor(token-meter): make context occupancy durable projection state

Replace the transient `session/model-request` mux frame with ordinary durable
session state. Occupancy now rides two last-wins projection fields instead of a
non-replayable frame that needed removal tombstones and cross-stream fencing.

The frame was the only non-replayable class on the mux stream. Because host and
mux are independent SSE streams with no cross-stream order, a request emitted
before a removal could arrive after `host/session-removed`, and a legitimate
request for a new lifecycle reusing the same id could be fenced by a late
removal. Fixing that needed a lifecycle generation on every frame; the frame
itself was the problem.

Removed: the `session/model-request` frame and schema, the `agent/model-request`
core event, the ApiProxy measurement point, the client-side telemetry map and
removal tombstone, and the synthetic `cancelled` open error used to signal
reconnect through the error channel.

Added: `request/context`, a log-only session event recording the
registration-bound capacity of the route a request resolved to, appended beside
`request/header` from the lookup that already prepared the call and skipped when
the route is unchanged. Capacity stays out of `EpochHeader` because it is
adapter metadata about a route, not an input the request was built from, so it
must not join request reconstruction or header equality.

The `contextPressure` projection pairs the newest provider-reported prompt size
with the newest recorded capacity. The two are deliberately not one atomic
request observation: switching models can pair a fresh capacity with the prior
route's pressure until the next request reports usage. The figure is a
user-facing reference, and this matches how the TUI status line has always
computed occupancy.

packages/host/apiproxy/package.json

@@ -60,19 +60,12 @@
     "zod": "^4.4.3"
   },
   "peerDependencies": {
-    "@deepseek-ai/dsh-token-meter": "^0.0.1",
     "cordis": "^4.0.0-rc.7",
     "@deepseek-ai/dsh-invariants": "^0.0.1"
   },
-  "peerDependenciesMeta": {
-    "@deepseek-ai/dsh-token-meter": {
-      "optional": true
-    }
-  },
   "devDependencies": {
     "@deepseek-ai/dsh-storage": "workspace:^",
     "@deepseek-ai/dsh-storage-domain": "workspace:^",
-    "@deepseek-ai/dsh-token-meter": "workspace:^",
     "cordis": "^4.0.0-rc.7",
     "@deepseek-ai/dsh-invariants": "workspace:^"
   }