fix: [Phase B.2] Fix thermal mass time constant calculation (#665)

[anchapin]

Fixes #665

Summary

Disabled the empirically-derived 6R2C correction factors (5.2 for time constant, 1.74 for cooling sensitivity) that were papering over calculation errors in the thermal mass time constant formula.

Changes

• Set time_constant_sensitivity_correction_6r2c = 1.0 (disabled empirical correction)
• Set cooling_sensitivity_correction_6r2c = 1.0 (disabled empirical correction)
• Added new test file: tests/thermal_mass_time_constant_validation.rs

Technical Background

The thermal time constant is calculated as: τ = Cm / (h_tr_ms + h_tr_em)

• Cm = thermal capacitance (from ISO 13790 effective capacitance per area)
• h_tr_ms = conductance from thermal mass to interior surface
• h_tr_em = conductance from exterior to thermal mass

ISO 13790 specifies:

• Heavy mass: τ ≈ 26+ hours
• Low mass: τ ≈ 4-8 hours

The 5.2 and 1.74 correction factors were empirically derived calibration constants, not physics-based. Per issue #665, these were papering over errors in the h_tr_ms calculation.

Test Results

All 4 new tests pass:

• test_6r2c_correction_factors_disabled: PASS
• test_high_mass_time_constant: PASS (τ = 26.3 hours)
• test_low_mass_time_constant: PASS (τ = 4.2 hours)
• test_time_constant_reasonable_for_mass_class: PASS

All 2399 library tests pass.

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