docs: add lesson on emulator Quick Boot SSD thrash and the fix

[timbortnik]

The Android emulator's default Quick Boot memory-maps guest RAM to ram.img, so the kernel continuously flushes dirty RAM pages to the SSD (~1.4 GB/s observed), independent of guest disk I/O. Documents the root cause, why it's not an app/ext4 bug, how to diagnose it (host vs guest /proc/diskstats; per-process tools undercount mmap writeback), and the fix: QuickbootFileBacked = off in ~/.android/advancedFeatures.ini.

Summary by CodeRabbit

• Documentation
  • Added a new lesson warning that Android Emulator Quick Boot can generate heavy host disk writes and quickly wear SSDs.
  • Clarified that the issue comes from host-side emulator snapshot writeback, not from the app or filesystem.
  • Added a verified workaround to disable file-backed Quick Boot on development machines.
  • Included guidance for spotting the issue and avoiding confusion between host and guest disk activity.

[coderabbitai]

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ℹ️ Recent review info

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Run ID: f48a2ccd-c493-472b-97a6-1f87d8b788e7

📥 Commits

Reviewing files that changed from the base of the PR and between 0a88ac1 and 5cd9663.

📒 Files selected for processing (1)

• docs/LESSONS_LEARNED.md

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📝 Walkthrough

Walkthrough

A new lesson is added to docs/LESSONS_LEARNED.md documenting that the Android emulator's Quick Boot (file-backed RAM) feature can cause sustained high SSD write activity on the host due to RAM-snapshot writeback, with a mitigation via QuickbootFileBacked = off in ~/.android/advancedFeatures.ini.

Changes

Emulator Quick Boot SSD Wear Lesson

Layer / File(s)	Summary
New lesson: Emulator Quick Boot SSD wear docs/LESSONS_LEARNED.md	Documents the root cause (mmap guest RAM with shared file-backed writeback to ram.img), mitigation (QuickbootFileBacked = off), and host vs. guest diskstats/vmstat guidance to correctly identify the write source.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly matches the change: a docs update about Android emulator Quick Boot causing SSD writes and the mitigation.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch docs/emulator-ssd-writes

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