[codex] Restore legacy analysis result fields

[Arcadia-1]

Summary

• Add legacy spectrum metric aliases such as sndr_db, sfdr_db, and noise_floor_db while preserving the current *_dbc / *_dbfs keys.
• Add fitted_sine to analyze_error_phase_plane() results as a compatibility field for older callers.
• Make compute_spectrum() return undefined distortion metrics as nan instead of raising ZeroDivisionError when the input has no signal peak.

Issue coverage

Partially addresses #28.

This resolves the low-risk compatibility/result-field failures and the zero-signal spectrum crash. The remaining #28 failures are still intentionally left open:

• calibration numeric tolerance/weight-recovery assertions need algorithm or test-contract review;
• test_verify_spec_plot_phase.py expects older tuple returns from polar APIs, while the current documented API returns dictionaries.

Validation

• MPLBACKEND=Agg uv run --with pytest pytest tests/unit/calibration/test_verify_estimate_frequencies.py::test_freq_estimation_shuffled -q -> 1 passed
• MPLBACKEND=Agg uv run --with pytest pytest tests/unit/calibration/test_verify_calibration_lite.py::test_calibration_lite tests/unit/aout/test_analyze_error_phase_plane.py::test_analyze_error_phase_plane_basic tests/unit/aout/test_verify_spec_plot.py tests/unit/fundamentals/test_nsd_snr_conversions.py tests/unit/fundamentals/test_verify_alias.py tests/unit/test_cap2weight.py -q -> 20 passed
• MPLBACKEND=Agg uv run --with pytest pytest tests/unit/models/ tests/unit/spectrum/ -q -> 581 passed
• git diff --check

A broader #28 representative run still has 5 known failures after this patch, matching the remaining issue scope described above.