Fix the macOS crash at import: defer config.py annotations

First real run on a Mac died before the dialog could open:

    config.py line 15: CELL_UM_OVERRIDE: float | None = None
    TypeError: unsupported operand type(s) for |: 'type' and 'NoneType'

KiCad macOS bundles Python 3.9, where PEP 604 unions in annotations
are evaluated at import time unless deferred - config.py was the one
annotated module without the __future__ import (errors.py has no
annotations, __init__.py is empty).

A new tripwire test walks every shipped module with ast and fails if
a file uses annotations without deferring them, so the next module
added (progress.py was born only last week) cannot regress this
silently while the Windows suite stays green.

Verified for real this time, not audited: the full suite passes under
CPython 3.9.25 with the exact stack a Mac venv resolves (numpy 2.0.2,
scipy 1.13.1, matplotlib 3.9.4, PySide6 6.10.0, pyamg 5.2.1) - 137
passed.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

fill_resistance/config.py

@@ -2,6 +2,9 @@
 
 A future version may read overrides from <project>/fill_res_config.json.
 """
+from __future__ import annotations   # KiCad's macOS Python is 3.9: without
+                                     # this, `float | None` annotations are
+                                     # evaluated at import and crash there
 
 # --- Grid sizing ---
 # Benchmarked on the VOUT+ plane (147x59 mm): R changes < 0.3% from