docs: add reporting model guide

[vku2018]

Summary

• add reporting/docs/REPORTING_MODEL.md documenting the HealthScore weights, component defaults, clamping, report roll-up, and DataAvailability semantics
• cover the Missing > Partial > Complete aggregation rule and the MAX_DEP_PAGES / DEP_PAGE_LIMIT cap behavior
• link the contract-local model guide from reporting/README.md

Closes #1022.

Validation

• PASS: source/doc keyword check for HealthScore, DataAvailability, component fields, page caps, and README link
• PASS: related link targets exist (docs/HEALTH_SCORE.md, docs/reporting-check-dependencies.md, reporting/docs/FAMILY_SPENDING_REPORT.md)
• PASS: git diff --cached --check
• BLOCKED: cargo --version / Rust doc/test commands cannot run in this environment because cargo is not installed