Add impact analysis, test coverage, and circular dependency endpoints to arch-docs

[jonathanpopham]

Summary

Integrate three new Supermodel analysis endpoints into the arch-docs pipeline so the generated site includes impact analysis, test coverage, and circular dependency data alongside the existing architecture documentation.

New Endpoints

POST /v1/analysis/impact

• Blast radius for files and functions — direct dependents, transitive dependents, affected entry points, risk score
• Accepts optional targets query param and diff file
• Add impact data to entity pages: risk level pill, blast radius stats, affected functions list

POST /v1/analysis/test-coverage-map

• Static test coverage via call graph reachability from test files (no test execution required)
• Per-function tested/untested status with confidence levels, per-domain coverage percentages
• Add coverage data to entity pages: tested/untested pill, test file links, coverage percentage on domain pages

POST /v1/analysis/circular-dependencies

• Tarjan's SCC cycle detection at file level with severity scoring
• Breaking suggestions for resolving cycles
• Add cycle data to entity pages: "In Cycle" pill on files that participate, cycle detail with Mermaid diagrams

Implementation

1. Call all three endpoints in parallel with the existing /v1/graphs/supermodel call (same zip upload, separate idempotency keys)
2. Save response JSON alongside graph.json
3. Extend graph2md input or post-process markdown to inject analysis data into entity frontmatter and body sections
4. Add new taxonomies: test_coverage (Tested/Untested), impact_level (Low/Medium/High/Critical), dependency_health (Clean/In Cycle)
5. Add D3 visualizations for impact radius and cycle diagrams