docs(windows): define crash durability boundary

[liugddx]

Summary

Document the Windows crash and durability boundary after the real-process recovery gates landed in #2509.

Clarify which guarantees come from SQLite WAL + synchronous=FULL and synchronized file payloads, and explicitly distinguish those guarantees from POSIX-equivalent parent-directory durability during sudden system power loss.

Refs #2142

Verification

• git diff --check upstream/main...HEAD
• Confirmed the current storage implementation skips directory fsync on Windows.
• Confirmed the runtime, session metadata, and long-term memory SQLite schemas use WAL with synchronous=FULL.
• Biome intentionally ignores Markdown files, so no Markdown formatter was run.

Checklist

• Tests cover the change and fail without it
• Lint, format, typecheck and the affected suites pass locally

Does this PR entail a change in behavior?

• Yes - described under Summary above
• No