docs: remove methods/statistical_models page; rescue WASP2 contracts to user_guide/analysis

[Jaureguy760]

Summary

Removes `methods/statistical_models.rst` (178 lines). The four WASP2-unique contracts flagged in the earlier sophia decision-auditor + co-scientist hypothesis-critic review are preserved as a compact "Defaults and contracts" section (~25 lines) in `user_guide/analysis.rst`.

Methods section is now a single page: `mapping_filter.rst`.

What's preserved in user_guide/analysis.rst

1. `pseudocount=1` and `min_count=10` defaults
2. Profile-LRT ρ convention (ρ held at null-MLE, df=1) — the PR docs: reduce text volume (~47%) + fix scientific correctness issues #99 scientific-correctness fix
3. ρ bounds `(1e-6, 1-1e-6)` and linear-dispersion logit clip `±10`
4. BH-NaN trap warning

Plus a pointer to `methods/mapping_filter.rst` for the canonical filter contract that applies to both remap and count steps.

Cross-refs updated (6)

• `index.rst` toctree entry removed
• `faq.rst` → `user_guide/analysis`
• `tutorials/bulk_workflow.rst` (2 occurrences) → `user_guide/analysis`
• `tutorials/single_cell_workflow.rst` → `methods/mapping_filter`
• `methods/mapping_filter.rst` → `user_guide/analysis`

Verification

• Sphinx build clean; `grep` confirms zero orphan `:doc:` refs
• CI: ruff + Rust + pytest 3.10/3.11/3.12 + CodeQL

Result

• Methods section: 2 pages → 1 (−178 lines)
• docs/source total: ~2,490 → ~2,310 lines (~−63% cumulative since session start)