Fix IMPA Hall gain sign and synchronize code defaults with vest.yaml (#624, #625)

[HongSik-Yun-Fusion]

Resolves #624 and #625.

Summary of Changes

1. Gain Sign Discrepancy (The IMPA Hall gain default has the opposite sign to the configuration and the documentation #624):
   • Corrected canonical Hall gain default from +2/15 to -2/15 (-0.13333333333333333 T/V) in ImpaProcessingConfig.gain and _resolve_gain fallback to match the DAQ wiring datasheet, docstrings, and vest.yaml.
2. Threshold & Offset Defaults (Three IMPA code defaults disagree with vest.yaml, and one configured threshold is never read #625):
   • Synchronized max_normalized_rmse default to 0.15 across grade_impa_quality, process_impa, and process_impa_shot to match vest.yaml quality gate.
   • Synchronized bz_radial_offset default to 0.01 (m) in process_impa and process_impa_shot to reflect physical probe head geometry.
3. Unread Parameter Clean-up (Three IMPA code defaults disagree with vest.yaml, and one configured threshold is never read #625):
   • Removed unread and unused TfWindowCriteria.max_relative_noise parameter from TfWindowCriteria, _window_criteria, and vest.yaml.
4. Test Suite:
   • Updated synthetic test fixtures in test/test_impa_processing.py to use canonical negative gain.
   • Added unit tests in test/test_impa_processing.py and test/test_impa_mapping.py verifying code defaults and YAML-configured parity.