Add behavior input binding contract

[MaxGhenis]

Fixes #510

Summary

• Add frozen, JSON-round-trippable behavior input binding models.
• Resolve Microcosm-owned population columns by stable entity ID while preserving nulls.
• Document the ownership boundary: PolicyEngine consumes non-legal population flags; Axiom remains documentary-only.

Explicit non-goals

No behavior or take-up formula, no takes_up_* concept, no Simulation integration, no Axiom behavior, no Belgian formula, and no numerical output change.

Verification

• 17 focused contract tests passed on the final tree.
• 36 Belgium/labor-supply tests passed with 4 expected source-stack skips, and 73 dataset/model/extra-variable regression tests passed before the issue-only final commits.
• Ruff format/check passed for all 197 files.
• Focused mypy and Towncrier checks passed.
• Architecture Markdown parsing and git diff checks passed.

Full docs remain an environment-only residual check because Quarto cannot write its Sass cache here; the changed Markdown parses cleanly. This remains a draft for frozen-head review and must not be merged yet.