feat: Extend attribution module with q2 humidity support

Add multi-variable attribution capability following the same Shapley
decomposition framework used for T2. The q2 (2m specific humidity)
attribution uses latent heat flux (QE) and the scale factor gamma =
1/(rho*Lv) to decompose humidity differences into physical mechanisms.

New functions:
- attribute_q2(): Decompose q2 differences between two scenarios
- diagnose_q2(): Automatic anomaly detection for humidity
- attribute(): Generic dispatcher for T2/q2
- diagnose(): Generic dispatcher for T2/q2
- _cal_gamma_humidity(): Scale factor for latent heat
- _cal_r_eff_humidity(): Back-calculate moisture resistance

The design maximises code reuse - both T2 and q2 follow the same
flux-gradient physics pattern (X = X_ref + r * F * gamma).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: sunt05

src/supy/util/__init__.py

@@ -58,10 +58,18 @@
 from ._spinup import get_spinup_state
 
 from ._attribution import (
+    # Variable-specific functions
     attribute_t2,
+    attribute_q2,
     diagnose_t2,
+    diagnose_q2,
+    # Generic dispatchers
+    attribute,
+    diagnose,
+    # Result container
     AttributionResult,
     T2Attribution,
+    Q2Attribution,
 )
 
 # from ._config import SUEWSConfig, init_config_from_yaml