wc-clean-sheet-model — clean-sheet probability + the stack correlation

Implements the clean-sheet and defensive-returns piece of footballfantasy/context/frameworks/scoring-rules.md, and supplies the cs_corr_bonus term that fitness-function.md adds and that the Clean-Sheet Spine (BB2) in building-blocks.md exists to capture. Defensive points are lumpy and team-correlated: the GK and the whole back line keep a clean sheet together or concede together. So a 3-defender-plus-GK stack from one strong defence is not four independent picks — it is one bet with an amplified ceiling and a shared floor. This skill is where that correlation is priced. Every defensive EV in the system traces back to the p_cs and expected_ga it emits; downstream agents read the signal, they do not re-derive it (signal-framework.md).

The two jobs:

1. Per-team clean-sheet probability and expected goals against for the specific fixture — the marginal building block of every GK/DEF/MID defensive return.
2. The joint distribution of a same-team stack — the correlation bonus (and its θ-dependent sign) that makes BB2 a coherent module and the Clean Sheet Shield chip a natural partner.

Workflow