perf: defer + cache cross-package index resolution off the first frame

[pchmn]

Summary

• Defer phase-B resolution off the first frame. The three feature indexes (AppIndex, ShortcutIndex, SettingsPageIndex) resolved other packages' labels/resources via PackageManager from Application.onCreate, contending with the first-frame composition on ART locks (ResourcesManager#getResources was the sole slow_start_reason). That work now runs from MainActivity after the first content draw, via a ViewTreeObserver.OnDrawListener + decorView.post (a Choreographer vsync count fired off-by-one — see ADR-0009).
• Disk-cache all three indexes (Option 2). ShortcutIndex + SettingsPageIndex now persist their entries (*IndexCacheRepository), like AppIndex. Cold start hydrates (phase A) before the first frame with zero getResources, so typed results and the blank-state recents render immediately — this fixes recent shortcuts/settings popping in late (they stale-filter against the live index; see ADR-0008).
• Pre-warm the GoogleSans device font (warmUpGoogleSans) so the first text layout doesn't pay the DeviceFontFamilyName lookup during the heavy post-first-frame composition (which queues WINDOW_FOCUS_CHANGED and delays the IME).
• Trustworthy benchmark. BootReceiver self-skips on the .benchmark build — macrobenchmark's StartupMode.COLD re-delivers BOOT_COMPLETED every iteration, which otherwise ran AppIndex.refresh during the first frame and inflated TTID uniformly.
• Result (Pixel 9, clean run): time_get_resources 376 → 111 ms, no-AOT cold-start floor 240.7 → 178.5 ms; Baseline ≈ None (~177 ms) — the path is now light enough that AOT barely matters. Docs updated (AGENTS.md, ADR-0008/0009, docs/performance-analysis.md).

Test plan

• ./gradlew compileDebugKotlin
• ./gradlew :benchmark:connectedBenchmarkReleaseAndroidTest — clean run, no BOOT_COMPLETED/phaseB.enumerate before TTID
• Recent shortcuts/settings render on the first frame (warm cache) on a real device
• First-ever launch (empty caches) still usable within the typing lead time

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