Support native Windows in the Langfuse backend and hook entrypoint #205

Motivation

Run the darrow-observability-langfuse backend and its registered Codex hooks on native Windows. Track this separately from the shell-test conversion in PR #204.

The current plugin has two concrete portability blockers:

• hooks/hooks.json invokes bash "${PLUGIN_ROOT}/hooks/stop.sh"; there is no native Windows launch path.
• backend/src/darrow_observability_langfuse/delivery.py and sidecar.py import Unix-only fcntl and use flock, so installing Bash alone does not make the backend work with native Windows Python.

The packaged hook execution tests skip Windows, and the observability fresh-install CI job runs only on Ubuntu. The README describes the launcher as portable without stating this native Windows limitation.

Acceptance criteria

• The installed plugin's registered hooks can launch the backend and capture/export telemetry on native Windows without requiring WSL or Bash.
• Backend delivery and sidecar locking work on native Windows while preserving their existing concurrency guarantees.
• Native Windows verification covers a fresh installed plugin and the registered hook launch path, rather than skipping those checks.
• Documentation states the supported platforms and their actual prerequisites.