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/context-meter.spec.tsx

@@ -89,6 +89,18 @@ describe('ContextMeter', () => {
     expect(panel.textContent).toContain('~0 / 128K')
   })
 
+  it('reads the ring from the projected figure so a compaction shows at once', () => {
+    // Same provider sample, a surface a compaction just shrank: the ring must
+    // follow the projection rather than the sample it is anchored to.
+    const view = meter({
+      contextPressure: { pressureTokens: 32_000, projectedTokens: 3_000, contextWindow: 128_000 },
+      contextBreakdown: BREAKDOWN,
+    })
+    const trigger = view.getByRole('button', { name: '上下文已用 2%' })
+    fireEvent.click(trigger)
+    expect(view.container.querySelector('[role="dialog"]')!.textContent).toContain('~3K / 128K')
+  })
+
   it('omits the composition rows while the contextBreakdown projection is absent', () => {
     const view = meter({ contextPressure: { pressureTokens: 32_000, contextWindow: 128_000 } })
     fireEvent.click(view.getByRole('button', { name: '上下文已用 25%' }))