Add periodic buffer persistence to protect against unexpected termination

[sgeraldes]

Summary

This PR adds periodic buffer persistence to protect against unexpected termination (BSOD, power failure, Task Manager kill).

Fixes #19804

The Problem

Previously, terminal buffer content was only persisted on graceful exit:

• At end of message loop (normal quit)
• On WM_ENDSESSION (graceful OS shutdown)

The 5-minute persistence timer only saved layout (tabs, panes, positions), not buffer content. This meant:

What Gets Saved	When	Survives Unexpected Termination
Tab layout	Every 5 minutes	✅ Yes
Buffer content	Only on graceful exit	❌ No

Users running long-lived sessions (Claude Code, SSH, development environments) would lose hours of terminal history on crash, even though tabs appeared to be "restored."

The Solution

• Extract buffer persistence into a new _persistBuffers() function
• Call _persistBuffers() on the existing 5-minute persistence timer
• Refactor _finalizeSessionPersistence() to use the shared function

Now buffer content is saved every 5 minutes alongside layout.

Validation Steps Performed

• Code review for correctness
• Verified refactored _finalizeSessionPersistence() maintains cleanup behavior
• Confirmed _persistBuffers() respects FirstWindowPreference::PersistedLayoutAndContent setting

PR Checklist

• Closes Feature Request: Periodic Buffer Persistence for Crash Recovery #19804
• Tests added/passed (N/A - existing persistence tests cover this code path)
• Documentation updated (N/A - no user-facing changes, existing setting controls behavior)
• Schema updated (N/A - no new settings)

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🤖 Generated with Claude Code

[sgeraldes]

@microsoft-github-policy-service agree

[sgeraldes]

Had to add a second commit to appease the spell checker gods. 📝

The irony here is chef's kiss — Microsoft's own CI spell checker flagging "BSOD" as an unknown word. You know, Blue Screen of Death... the thing Windows has been (in)famous for since Windows 3.1?

I get it, we're trying to move past that reputation, but denying its existence in the dictionary feels like revisionist history. 😄

The term is now safely enshrined in expect.txt where it belongs — right between Bspace and BTNFACE, as Ballmer intended.

[DHowett]

The irony here is chef's kiss — Microsoft's own CI spell checker flagging "BSOD" as an unknown word. You know, Blue Screen of Death... the thing Windows has been (in)famous for since Windows 3.1?

I get it, we're trying to move past that reputation, but denying its existence in the dictionary feels like revisionist history. 😄

I appreciate your contribution, but don't be weird about it. It's not in the dictionary because it is not used in this codebase. The spell checker is maintained by a non-Microsoft person, hosted in a non-Microsoft repository, as non-Microsoft property, and the dictionaries only get updated as needed when somebody wants to use an unusual term of art.