Fix signal-generator nonideality semantics

[Arcadia-1]

Summary

• fix apply_reference_error so zero droop is an identity transform in the configured DC reference frame
• clarify RA gain-error semantics by preserving legacy coarse-path behavior and adding explicit residue_path mode
• add focused siggen nonideality tests for reference droop, RA path semantics, validation, and dynamic legacy behavior

Verification

• uv run --with pytest pytest tests/unit/siggen/test_nonidealities.py -q
• uv run --with pytest pytest tests/unit/siggen -q
• uv run python src/adctoolbox/examples/03_generate_signals/exp_g06_sweep_dynamic_nonlin.py
• uv run python src/adctoolbox/examples/03_generate_signals/exp_g07_sweep_interferences.py
• uv run --with pytest pytest tests/integration/test_user_guide_skill_examples.py -q
• uv run --with pytest pytest -q
• git diff --check

Closes #52
Closes #53