perf(web-ui): shorten startup handoff and harden pet sync

[limityan]

Summary

• Shorten the startup splash handoff: minimum hold 900ms -> 650ms, exit animation 0.32s -> 0.24s, fallback 450ms -> 350ms.
• Add a single startup_overlay_hide_start trace point so startup reports can separate "ready to hide" from "overlay removed".
• Fix performance runner sampling flags (--samples / --iterations) and isolate long-session interaction matrix data under per-run E2E roots.
• Harden Agent companion pet lifecycle without changing its loading path: the pet component remains on the companion-window startup path, startup sync reads latest AI experience settings at execution time, stale show/hide requests no longer emit stale settings, the pet sends a ready handshake after listeners are registered, and inactive disabled / input settings hide an incorrectly-created pet window.

Performance Data

Release-fast Windows run against latest main baseline 99fd8448, 10 samples each. The claimed product-facing improvement is startup to main UI visible (startup_overlay_hidden). Agent companion changes are correctness hardening and are not counted as a separate performance optimization.

Scenario / metric	Before	After	Delta
Startup to main UI visible (startup_overlay_hidden, median)	1483.4 ms	1027.7 ms	-455.7 ms
Non-critical startup done (nonCriticalInitDoneMs, median)	1763.0 ms	1457.7 ms	-305.3 ms
Shell interactive ready (interactiveShellReadyMs, median guard)	626.5 ms	493.1 ms	-133.4 ms
Main shown to interactive (shownToInteractive, median guard)	211.6 ms	195.3 ms	-16.3 ms

Long-session guard on the final build (release-fast, core matrix): first open 170.6 ms, rapid switch 242.6 ms, first scroll 150.8 ms, resize-width 158.3 ms, all with blankEvents=0.

Risk / Scope

• This does not change LSP, ACP, remote, session hydration, or editor loading behavior.
• Agent companion pet may still start after the main shell is interactive, but the desktop pet is not deferred out of its own companion-window startup path. The latest config wins if startup sync races with settings or tray changes.
• The splash can disappear earlier, but still waits for workspaceLoading=false and AppLayout ready, keeps a 650ms minimum hold, and exits via a 240ms animation.
• The added startup trace is one phase entry only; pet lifecycle hardening adds no verbose logging in normal successful paths.
• The long-session matrix now writes generated data only to isolated E2E runtime roots and prunes old perf runs.

Validation

• pnpm run desktop:build:release-fast
• pnpm run type-check:web
• pnpm run lint:web
• pnpm --dir src/web-ui run test:run src/infrastructure/config/services/AgentCompanionWindowService.test.ts src/infrastructure/config/services/AIExperienceConfigService.test.ts src/app/startup/startupPerformanceContract.test.ts src/app/startup/startupOverlay.test.ts src/app/startup/startupPreload.test.ts src/flow_chat/services/AgentCompanionActivityBridge.test.ts src/flow_chat/utils/agentCompanionActivity.test.ts
• node tests/e2e/scripts/perf-coverage-contract.test.mjs
• pnpm run e2e:test:perf:startup-stability:release-fast -- --samples 10
• pnpm run e2e:test:perf:long-session-interactions:release-fast -- --profile core
• git diff --check