Add API/parameter reference and remove the broken spring-cart example

[dantzert]

README.md has a solid quick-start and a functionality overview, but the project lacks an API/parameter reference and worked examples. IDEA.md is a one-line pointer; docs/ contains only an internal issue_18_plan.md. The root holds a stray BAYESIAN_OPTIMIZATION.md that documents a feature not mentioned in the README.

Additionally, spring-cart-interactive.py opens with # not at all working, imports ipywidgets (which is not a declared dependency), and is never exercised by CI — so it's broken dead weight rather than a usable example.

Undocumented public surface (all live on main):

• optimization_method ("bayesian", "differential_evolution", "dual_annealing", "simulated_annealing", "direct", "brute") in delay_io_train.
• subsample_frac / subsample_seed, transform_dependent / transform_only, force_coef_constraints sign semantics, early_stopping_threshold.
• TransformCache (quantization + LRU stats).

Proposal:

• Add docs/ API reference (sphinx or mkdocs, docstrings as source of truth) covering parameters + semantics for each public function in modpods/__init__.py.

• Either fix spring-cart-interactive.py into a working, dependency-declared example, or remove it; add a small example script/notebook under examples/ that works in CI.

• Fold BAYESIAN_OPTIMIZATION.md into the docs (or remove it).

• Cost: Medium.

• Benefit: Medium (onboarding/adoption; surfaces the newer features so users don't only discover them via tests/).