Add signal-decomposition degradation analysis

[bmeyers]

Ports the CVXPY seasonal-trend decomposition prototype into a new
rdtools.signal_decomposition module and wires it end-to-end through
TrendAnalysis.

New module (rdtools/signal_decomposition.py):

• Public degradation() entry point returning (Rd_pct, Rd_CI, sd_trend_results)
• Three trend types: linear, pwl, monotone
• Four loss functions: l2, l1, huber, quantile
• Missing data handled natively via masked equality constraint
• Stability analysis: analyze_fit_stability, get_valid_endpoints, plot_stability
• Plotting/display: plot_decomposition, animate_degradation, format_degradation_report
• log_transform path via extract_degradation_rate_log

TrendAnalysis wiring (rdtools/analysis_chains.py):

• _signal_decomposition_degradation private method
• sensor_analysis / clearsky_analysis accept 'signal_decomposition' in analyses
  list and sd_kwargs passthrough
• plot_signal_decomposition_summary method

Dependencies:

• cvxpy >= 1.7.5 and spcqe >= 0.3.0 added as hard runtime dependencies
• scipy floor raised from 1.8.1 to 1.13.0 (required by cvxpy's transitive deps)
• cvxpy and spcqe excluded from check_dependencies.py min-pin enforcement
  (their transitive scipy requirement is incompatible with pinning an exact
  lower-bound version alongside scipy==1.13.0 on Python 3.10)

Tests and docs:

• rdtools/test/signal_decomposition_test.py (29 tests)

• Additions to analysis_chains_test.py (fixtures, plot tests, bad-case coverage)

• api.rst updated with Signal Decomposition section and new TrendAnalysis method

• Code changes are covered by tests

• Code changes have been evaluated for compatibility/integration with TrendAnalysis

• [ ] New functions added to __init__.py
  We are intentionally not doing this. The new function should be imported as rdtools.signal_decomposition.degradation / from rdtools.signal_decomposition import degradation to avoid clashes with the rdtools.degradation module.

• API.rst is up to date, along with other sphinx docs pages

• Example notebooks are rerun and differences in results scrutinized

• Updated changelog

[bmeyers]

Just finishing up examples and package dependency testing. Ready for general review. Open item: minimum scipy version support.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 78.02817% with 156 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.45%. Comparing base (b5eb6e2) to head (84267c0).

Files with missing lines	Patch %	Lines
rdtools/signal_decomposition.py	71.24%	132 Missing ⚠️
rdtools/degradation.py	84.55%	21 Missing ⚠️
rdtools/plotting.py	95.31%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #518      +/-   ##
==========================================
- Coverage   96.79%   92.45%   -4.34%     
==========================================
  Files          12       13       +1     
  Lines        2342     3050     +708     
==========================================
+ Hits         2267     2820     +553     
- Misses         75      230     +155     

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