fix(app): keep long chat streaming work bounded

Reuse unchanged history work while replies stream, without retaining every
window visited while reading older messages. Verify rendering cost in the
browser and collection of obsolete windows through the model interface.

Co-authored-by: Ivan Malison <IvanMalison@gmail.com>

docs/agent-stream-performance.md

@@ -58,12 +58,17 @@ So arrival sets a _target_ and the reveal rate is derived from the backlog inste
   through that hook; the web viewport once skipped it and history hosts of a live tool group went
   stale. A new field on `StreamLayoutItem` must be added to `areLayoutItemsEquivalent`, or sharing
   silently stops.
+- **Streaming cost must not grow with revealed history.** Reading older messages must not
+  make the next reply progressively more expensive to display. Reuse unchanged history work
+  while text arrives, and release obsolete window caches as the reader reveals more history;
+  window caches must stay bounded while their source timeline remains loaded.
 
 ## Measuring
 
 - **Smoothness (user-perceived):** `packages/app/e2e/browser/agent-stream-smoothness.spec.ts`, gated behind `PASEO_AGENT_STREAM_PERF_E2E=1`. Drives the mock provider's `bursty-stream` model and reports coefficient of variation of characters painted per frame (smoothness) plus p95 gap between visible updates (stalls). Both numbers are needed: a stalled stream is perfectly smooth.
 - **Reproducing bursty arrival:** the `bursty-stream` model in `mock-load-test-agent.ts` emits uneven runs of tokens separated by idle gaps. Burst sizes come from a seeded generator, so a run repeats exactly.
 - **Rate policy in isolation:** `computeRevealStep` in `packages/app/src/agent-stream/text-reveal.ts` is pure; `text-reveal.test.ts` covers convergence and burst flattening without a renderer.
+- **Per-delta cost on a long timeline:** `npx vitest bench --project unit src/agent-stream/stream-tick.bench.ts` from `packages/app` replays 200 deltas over a 2,000-item timeline through the pure model and layout pipeline, with the default mounted window and with history fully revealed. It measures the JS work per delta, not paint.
 
 Healthy numbers (2026-08, Expo web against a local dev daemon, real Claude Haiku agent, ~8.5s samples during active streaming). Setting `TEXT_REVEAL_HORIZON_MS` to 0 makes the reveal paint on arrival, which is how the baseline column was taken: