perf(web): reuse work log rows during streaming

[Bil0000]

Long threads rebuild their full work log for every streamed activity update. That reparsed unchanged activity payloads, cloned every output row, and changed row identities even when the source activities had not changed.

This change caches each immutable activity's derived work-log entry in a WeakMap, keeps collapse metadata private with a symbol, and returns collapsed entries without a second cloning pass. Unchanged activity rows now keep stable identities across append updates.

Benchmark

• Scenario: derive 20,000 ordered command activities, append one activity, then derive again.
• Before median: 163.6 ms.
• After median: 10.1 ms.
• Median change: 93.8% less time.
• Before p95: 275.5 ms.
• After p95: 22.9 ms.
• The committed regression requires the append update to finish within 100 ms and checks stable entry identity.

Verification

• pnpm --filter @t3tools/web typecheck
• vp lint apps/web/src/session-logic.ts apps/web/src/session-logic.test.ts --report-unused-disable-directives
• vp fmt --check apps/web/src/session-logic.ts apps/web/src/session-logic.test.ts
• vp test run src/session-logic.test.ts --project unit (77 tests)
• vp test run src/components/chat/MessagesTimeline.logic.test.ts src/components/chat/MessagesTimeline.test.tsx --project unit (77 tests)

Created with GPT-5.6 in the Codex harness through T3 Code.

Note

Memoize DerivedWorkLogEntry via WeakMap to reuse work log rows during streaming

• Caches the result of toDerivedWorkLogEntry in a module-level WeakMap keyed by activity object, so repeated derivations for the same activity return the same entry instance
• Moves the collapse key from the string field collapseKey to a symbol-keyed property [workLogCollapseKey], eliminating the final mapping step in deriveWorkLogEntries that previously stripped internal fields
• Renames activityKind to sourceActivityKind on DerivedWorkLogEntry; updates all readers in toolLifecycleCollapseMapKey, collapseDerivedWorkLogEntries, shouldCollapseToolLifecycleEntries, and deriveToolLifecycleCollapseKey
• Adds performance tests in session-logic.test.ts verifying identity reuse for unchanged entries and 20k-entry derivation within 100 ms
• Behavioral Change: DerivedWorkLogEntry no longer exposes activityKind or collapseKey; any external reader of these fields must switch to sourceActivityKind and [workLogCollapseKey]

^{Macroscope summarized 647e760.}

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Approvability

Verdict: Skipped

Macroscope did not run approvability analysis for this PR. Macroscope could not determine whether this PR modifies its approvability configuration, so the PR was not approved automatically. A PR that may change the rules that govern approval is never approved automatically.