Make private runtime files and history locking portable across POSIX and Windows

[codeforester]

Parent: #35

Goal

Give runtime files, directories, and history writes well-defined privacy and concurrency behavior on POSIX and Windows.

Scope

• Preserve owner-only mode enforcement on POSIX.
• Define the Windows privacy contract using user-profile locations and inherited ACLs; do not claim POSIX mode-bit semantics where they do not apply.
• Make private JSON and log writes safe when chmod/mode bits are ineffective on Windows.
• Harden the msvcrt history-locking path and add concurrent append tests.
• Keep the existing fcntl implementation and behavior unchanged on Unix-like systems.
• Document behavior when an application supplies a cache root outside the user profile.

Acceptance criteria

• No platform silently weakens the documented privacy guarantee.
• Concurrent history writers do not corrupt records on POSIX or Windows.
• Runtime creation and cleanup errors remain actionable.
• Tests cover both backends and preserve current Linux/macOS expectations.