perf(flow-chat): reduce historical session hydration stalls

[limityan]

Summary

• Reduce historical-session first-open and switch stalls by rendering the latest readable tail first and hydrating older history after the latest content is stable.
• Prevent rapid session switching from syncing/opening a stale session after a newer target becomes active.
• Add strict E2E guards for latest-anchor failures, post-visible loading/blank/scroll-jump regressions, and rapid-switch non-target content holds.
• Add native startup trace breakdown for tray/window creation so startup outliers are visible.

Status

Merged as b34a970c.

A post-merge review found one additional P2 interaction risk in the old-history window expansion scroll mapping. That follow-up fix is tracked separately in #1109 because this PR had already merged.

Performance Data

Measured with release-fast builds and the generated 80-turn mixed-visible historical-session fixture. Baseline is main at a824830a; after is this PR before merge.

Scenario	Before	After	Delta / result
First open long session, latest content usable	619.0 ms	419.4 ms	-199.6 ms (-32.2%)
First open long session, post-hydrate usable	3635.4 ms	3441.2 ms	-194.2 ms (-5.3%)
Warm reopen long session, latest content usable	351.5 ms	200.6 ms	-150.9 ms (-42.9%)
Rapid switch A->B->C, target latest usable	1165.2 ms	735.9 ms	-429.3 ms (-36.8%)
Latest-anchor failure count, first/warm open	1 / 1	0 / 0	Strict guard passes
Rapid-switch non-target content holds	4	0	Stale session content no longer remains visible after target open intent
Post-visible loading/blank/scroll-jump events	0 / 0 / 0	0 / 0 / 0	No regression captured

Startup was measured for visibility, but not claimed as solved by this PR because native window timing still shows environment variance.

Risk / Compatibility

• Startup is not treated as solved by this PR; native window creation/maximize still needs separate investigation.
• Remote session behavior is not intentionally changed by this PR.
• New trace points are startup/session E2E diagnostics and startupTrace phases, not high-volume production info logs.

Validation

• pnpm run desktop:build:release-fast
• pnpm run e2e:test:perf:release-fast
• BITFUN_E2E_PERF_POST_VISIBLE_INTERACTION=first-scroll pnpm run e2e:test:perf:release-fast
• pnpm --dir src/web-ui run test:run src/flow_chat/components/modern/VirtualMessageList.layout.test.ts src/flow_chat/services/FlowChatManager.test.ts src/flow_chat/store/FlowChatStore.test.ts src/flow_chat/services/openBtwSession.test.ts src/flow_chat/components/modern/ModernFlowChatContainer.history-state.test.tsx
• pnpm run type-check:web
• pnpm run check:repo-hygiene
• cargo check -p bitfun-desktop
• git diff --check gcwing/main...HEAD