[Raing5Days]

Summary

Profiling the Web UI streaming render path turned up four places where work ran
unconditionally on every batched flush (every 32ms while text streams). All four are
fixed here as one focused change set on the same hot path.

• TextChunkModule — newline coalescing re-scanned the whole accumulated response on
  every chunk: (currentContent + text).replace(/\n{3,}/g, '\n\n'). That is O(N) per
  chunk and O(N^2) over a long answer, on the main thread. Both sides of an append are
  already clean, so only the two-character join boundary plus the incoming chunk need
  cleaning. Recovered item content is cleaned once when reused, so the buffer invariant
  still holds.
• EventBatcher — getBatchedEventsLogPayload() ran before log.trace(), and the
  logger drops the entry after its level check. With the default level (WARN in
  production, DEBUG in dev) every flush allocated a full per-event map for a log line
  that was then thrown away. The payload is now built only when trace output can reach a
  sink; the slow-flush warning keeps its counts.
• modernFlowChatStore — dropped the immer middleware. The three setters only assign
  whole fields, never a draft, so produce() plus immer's deep freeze walked the entire
  session and virtual-item tree on every flush. Plain set() keeps the same reference
  semantics. clear() now also resets the needsUserAttention projection cache guard
  alongside the other cached fields.
• useConversationTimeline — the prewarm effect depends on [sources], which changes on
  every flush, so it re-walked the whole timeline including items already queued or
  parsed. A WeakSet of seen items skips them.

Fixes #

Type and Areas

Type: performance / refactor

Areas: web UI (src/web-ui/src/flow_chat)

Motivation / Impact

Streaming a long answer got progressively more expensive per flush, and part of that
cost bought nothing: a redundant full-text rescan, a log payload that was discarded, a
state-tree deep freeze nothing depended on, and a repeated timeline sweep.

User-facing effect: long streaming responses keep a steadier frame budget and the cost
per flush stops growing with the length of the conversation. No behavior, API, or
persisted-shape change.

Verification

Run on this branch, based on origin/main at e17e1803e:

• pnpm --dir src/web-ui run test:run src/flow_chat/services/flow-chat-manager/TextChunkModule.test.ts src/flow_chat/services/EventBatcher.test.ts src/flow_chat/store/modernFlowChatStore.test.ts → 109 passed
• pnpm --dir src/web-ui run test:run src/flow_chat/timeline/ → 24 passed across 8 files
• pnpm --dir src/web-ui exec tsc --noEmit -p tsconfig.json → clean
• pnpm --dir src/web-ui run lint → 0 errors (14 pre-existing warnings, none in the changed files)

TextChunkModule.test.ts did not assert newline coalescing directly before this change;
behavior equivalence there is covered by the existing streaming tests. A dedicated
"chunk boundary crosses 3+ newlines" case would be a good follow-up.

Reviewer Notes

Two points worth a careful look:

1. modernFlowChatStore no longer freezes its state. Nothing reads frozen state or hands
   an updater to setState on this store, and useFlowChatActions selects actions through
   useShallow, so it is unaffected. canvasStore keeps its immer middleware.
2. The newline fix relies on the invariant "the content buffer never holds a 3+ newline
   run". Both buffer writers go through the new helper, and recovered item content is
   cleaned once on reuse.

Remote scenarios: this touches the local Web UI render path only. No changes to remote
workspace transport, remote control, Peer Device Mode command handling, or the detached
dispatch protocol, and none of those paths consume these modules.

Checklist

• This PR is focused and does not include secrets, temporary prompts, generated scratch files, or unrelated artifacts.
• Relevant verification is recorded above, or skipped checks are explained.
• User-facing strings, docs, and locales are updated where applicable. (No user-facing copy changed, so nothing to update.)