FIX make file output safe across event loops

[Roman Lutz (romanlutz)]

Description

FileSink currently uses an asyncio.Lock, which becomes bound to the first event loop that contends on it. Reusing the same sink from a later asyncio.run(...) call can therefore raise a cross-event-loop RuntimeError.

This change moves serialization into the worker thread with threading.Lock, so the event loop remains non-blocking while one sink can safely serialize file writes across event loops. It also removes an unreferenced ecosystem SVG discovered during the audit and updates the output contributor guidance to match the locking design.

Tests and Documentation

• Added a deterministic two-event-loop, two-contender regression test using events rather than sleeps. It verifies exact write ordering, terminal completion, lock cleanup, non-duplicated output, and absence of an early second write.
• Full unit suite: 17,543 passed, 8 skipped.
• Final Crescendo module after syncing main: 84 passed.
• FileSink tests: 13 passed.
• Ruff check and format check passed.
• Full ty check pyrit passed with no diagnostics.
• uv lock --check, compileall, documentation TOC/orphan validation, and pre-commit hooks passed.
• JupyText was not run because no notebook source or notebook file changed.