perf: cache L in Constant adapter, skip Σ re-decomposition (closes #68)

[thomaspinder]

Summary

• Constant.build_pymc_latent now registers L as a pm.Deterministic alongside Sigma
• Constant.cholesky_at reads posterior["L"] directly instead of calling np.linalg.cholesky(Σ) on every invocation; this propagates to cholesky_path and forecast_cholesky_path which delegate to it
• Posterior gains one new variable (L); existing consumers (Sigma, B, intercept, sigma_sd, tril_offdiag) are byte-for-byte unchanged

Closes #68.

Test plan

• pytest tests/test_volatility.py -v — 20/20 pass, including both @pytest.mark.slow TestPosteriorEquivalence MCMC tests
• pytest -m "not slow" -q — 227 passed, 22 deselected, no regressions
• ruff check src/ tests/ — clean
• ruff format --check src/ tests/ — clean
• ty check — clean
• New TestConstantCholeskyAt::test_reads_cached_L_not_decomposed_sigma pins the "reads cached L, never re-decomposes Σ" contract by feeding deliberately-inconsistent L/Σ to the adapter
• TestPosteriorEquivalence::test_explicit_constant_matches_string_default extended to include L in the byte-for-byte equality check across volatility= default / "constant" / Constant()

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.4%. Comparing base (6e8aad4) to head (8ff239a).

Additional details and impacted files

@@          Coverage Diff          @@
##            main     #69   +/-   ##
=====================================
  Coverage   92.3%   92.4%           
=====================================
  Files         26      26           
  Lines       1127    1141   +14     
  Branches     107     111    +4     
=====================================
+ Hits        1041    1055   +14     
  Misses        65      65           
  Partials      21      21           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.