fix: mutable global state in _CUSTOM_CHECKS is not thread-safe

[rororowyourboat]

Problem

packages/gds-framework/gds/helpers.py:170 — The _CUSTOM_CHECKS module-level list is mutated by the @gds_check decorator at import time. No locking or thread-safety mechanism exists.

Same concern applies to GDSSpec (spec.py) — a mutable registry with no concurrent access guards.

Impact

• Parallel test runners could register checks multiple times
• Multi-threaded applications building specs concurrently could corrupt state
• Not a problem today (single-threaded usage), but a latent issue

Options

1. Document that _CUSTOM_CHECKS and GDSSpec are not thread-safe (cheapest)
2. Add threading.Lock guards to mutation points
3. Use contextvars for per-context isolation