fix(web): let the context meter see a compaction

The composer ring, percentage, and `~used / capacity` header read
`contextPressure.pressureTokens`, which moves only when a request reports
usage. Compaction reports none — compact-basic summarizes through a direct
`ctx.llm.stream()` call and appends only its own `compact/*` records plus the
replacement `user/message` — so the meter was frozen across the one action
taken to change it. Driving a real `compactNow` through the agent loop:

    BEFORE compact:  ring=4%  header=~4227/100000  rows=[18, 0, 4365]
    AFTER  compact:  ring=4%  header=~4227/100000  rows=[18, 0,  286]

The composition rows fell 93%; the ring did not move, and would not until an
entire further turn completed. The panel then contradicted itself by more than
an order of magnitude at exactly the moment a reader opens it.

`contextPressure` now also publishes `projectedTokens`: the provider sample
plus the heuristic repricing of everything the surface gained or lost since
that sample, clamped at zero, folded through the shared `surface-fold.ts`. The
sample is stamped before the same event joins the surface, so an
`assistant/message` anchors against the surface its own request carried. Only
the delta is estimated, so the figure stays provider-anchored — the estimator's
CJK and JSON-schema underpricing stays out of the occupancy number — while
reacting the moment content lands or a span is shadowed. Same run after:

    BEFORE compact:  ring=4%  header=~4323/100000  (pressure=4227, projected=4323)
    AFTER  compact:  ring=0%  header=~ 244/100000  (pressure=4227, projected= 244)

`contextOccupancy` prefers the projected figure and falls back to the bare
sample, so a projection restored from a pre-field checkpoint degrades to the
old behavior rather than disappearing. `stateVersion` moves to 3.

packages/client/ui-conversation/tests/chat-stats-bash-sample.spec.tsx

@@ -220,16 +220,21 @@ describe('StatsLine', () => {
       .toBe('Cache hit 90%| Input 100 tok · Output 5 tok')
   })
 
-  it('computes context occupancy only when both pressure and capacity are known', () => {
+  it('computes context occupancy only when both a numerator and capacity are known', () => {
+    // The projected figure wins: it is the provider sample carried forward over
+    // the surface's movement, so a compaction shows without waiting a request.
+    expect(contextOccupancy({ pressureTokens: 32_000, projectedTokens: 6_000, contextWindow: 128_000 }))
+      .toEqual({ percent: 5, usedTokens: 6_000, contextWindow: 128_000 })
+    // A log whose projection predates the field still reads its bare sample.
     expect(contextOccupancy({ pressureTokens: 32_000, contextWindow: 128_000 }))
-      .toEqual({ percent: 25, pressureTokens: 32_000, contextWindow: 128_000 })
-    // Pressure without capacity has no denominator; capacity without a provider
-    // sample has no numerator yet, rather than a synthetic 0%.
+      .toEqual({ percent: 25, usedTokens: 32_000, contextWindow: 128_000 })
+    // A numerator without capacity has no denominator; capacity without a
+    // provider sample has no numerator yet, rather than a synthetic 0%.
     expect(contextOccupancy({ pressureTokens: 32_000 })).toBeNull()
     expect(contextOccupancy({ contextWindow: 128_000 })).toBeNull()
     expect(contextOccupancy(undefined)).toBeNull()
-    // Capacity and pressure are independent last-wins fields, so a model switch
-    // can pair a smaller new window with the previous route's larger prompt.
+    // Capacity and the sample are independent last-wins fields, so a model
+    // switch can pair a smaller new window with the previous route's prompt.
     expect(contextOccupancy({ pressureTokens: 300_000, contextWindow: 128_000 })?.percent).toBe(100)
   })