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/client/runtime/tests/queue-store.spec.ts

@@ -93,13 +93,6 @@ describe('queue retirement (host queuedMirror rules)', () => {
     expect(session.getSnapshot().queue).toHaveLength(1)
   })
 
-  it('an unrelated durable event leaves the queue unchanged', () => {
-    const session = makeSession()
-    session.handleMuxEnvelope(rid('e1'), queuedFrame('留', 'p-1'))
-    session.handleMuxEnvelope(rid('e2'), { type: 'session/event', sessionId: SID, event: ev.user(0, 'unrelated') })
-    expect(session.getSnapshot().queue.map(row => row.key)).toEqual(['p-1'])
-  })
-
   it('steering/message drains the source-matched steering row only', () => {
     const session = makeSession()
     session.handleMuxEnvelope(rid('e1'), queuedFrame('普通', 'p-1')) // idle → non-steering