feat(web): show durable token usage and context occupancy in the stats line

The chat stats line took its token totals from the loaded conversation nodes,
so paging changed them and compaction erased the billing behind replaced
content. It also had no way to show context occupancy: the numerator and
capacity never reached the browser.

Both now come from token-meter session projections read through the standard
useProjection seat. Window nodes keep supplying turn and step counts plus LLM
and tool wall times, which are correctly window-scoped facts about what is on
screen; accounting no longer comes from there.

`tokenUsage` supplies billing and cache hit. `contextPressure` supplies
occupancy, pairing the newest provider-reported prompt size with the newest
capacity recorded by `request/context`. Deployments without token-meter drop
the token groups; a route whose adapter advertises no capacity drops the
occupancy group rather than rendering a placeholder.

Occupancy is deliberately approximate: the numerator and capacity are
independent last-wins fields, not one atomic request observation, so switching
models pairs a fresh capacity with the prior route's pressure until the next
request reports usage. It is a user-facing reference figure that nothing in the
harness makes decisions from, and it matches how the TUI status line has always
computed occupancy. The Agent Note and token-meter README state this as a
decision, including why the atomic alternative was implemented and rejected, so
it is not re-litigated as a defect.

Snapshot delta is one added `Context N% of 128K` segment across eight web
goldens; the preceding commit absorbed master's pre-existing golden drift.

docs/core-data-structures/session.md

@@ -91,6 +91,16 @@ interface SessionEventMap {
    * It is log-only; the latest snapshot reconstructs the request header.
    */
   'request/header': { header: EpochHeader; reason: RequestHeaderReason }
+  /**
+   * Registration-bound context capacity for the route a request resolved to,
+   * appended inside its step beside `request/header` and only when the route
+   * or capacity differs from the last record. It is log-only and deliberately
+   * NOT part of {@link EpochHeader}: capacity is adapter metadata about a
+   * route, not an input the request was built from, so it must not participate
+   * in request reconstruction or header equality. Absent for a route whose
+   * adapter advertises no capacity.
+   */
+  'request/context': RequestContext
 }
 ```
 
@@ -141,6 +151,26 @@ interface EpochHeader {
 
 Canonical form represents an empty system prompt or tool list as an absent field, matching how requests are built. Legacy v0 logs containing the removed `request/header-delta` event or its full-snapshot `fallback` reason are rejected at seed, append, and persistence-load boundaries rather than replayed incompletely.
 
+### The route capacity event: `request/context`
+
+The context window of the route a request resolved to is separate logged state, appended beside `request/header` inside the same step and only when the provider, model, or capacity differs from the previous record. It stays outside `EpochHeader` because that type is the reconstruction contract compared field-wise by `headerEquals`: capacity describes a route, not a request input, so folding it in would let a capacity change register as a request-envelope `change` and would pull adapter metadata into the loop's reconstruction invariant. Like `request/header`, it is not a `SurfaceEventType` and produces no LLM message. `session.requestContext()` folds the latest record incrementally. A route whose adapter advertises no capacity appends nothing, which consumers read as "capacity unknown".
+
+```ts type-equiv
+/**
+ * Registration-bound context capacity of one resolved model route. Adapter
+ * metadata about a route rather than a request input, which is why it lives
+ * outside {@link EpochHeader}.
+ */
+interface RequestContext {
+  /** Registered provider route the capacity was resolved through. */
+  provider: string
+  /** Provider-owned model id the capacity belongs to. */
+  model: string
+  /** Maximum combined request and response context in tokens. */
+  contextWindow: number
+}
+```
+
 ## `SessionEvent<T>` — one log entry
 
 A proper discriminated union over `type` (not independent `type`/`data` unions), so `switch (event.type)` narrows `event.data` without casts. `seq` is the monotonic position in the log (`seq = log.length`); `time` is epoch ms.
@@ -388,6 +418,14 @@ declare class Session {
    * @returns the folded header, or undefined when no header event exists yet.
    */
   requestHeader(): EpochHeader | undefined;
+  /**
+   * The route capacity in force after the log's last `request/context` event —
+   * what the NEXT request deduplicates against — or undefined before any such
+   * record. Maintained incrementally like {@link requestHeader}, so a per-step
+   * read costs O(new events).
+   * @returns the folded capacity record, or undefined when none exists yet.
+   */
+  requestContext(): RequestContext | undefined;
   /**
    * Derive the LLM message history by walking the ordered sequences of
    * message-producing events maintained by `surfaceOp` markers. The