fix: route sibling RFP links correctly

[arunimshukla]

Summary

I fixed bare sibling links between RFP pages resolving to /use-cases/... instead of /rfps/....

RFP source files use valid same-directory links such as rfp-trust-assurance.md. Because RFPs are intentionally outside the graph and the bare filename has no directory component, the renderer previously fell through to the first empty-prefix content type and produced a use-case route.

The change:

• recognises bare rfp-*.md links before graph candidate matching
• routes them to the existing /rfps/<slug>/ pages
• preserves fragments and queries
• keeps existing directory-qualified RFP links and other content links unchanged
• adds regression coverage for the affected RFP targets

Validation

I reproduced the incorrect routing for the four affected RFP targets and verified the fix against all seven current sibling-link occurrences in the map content.

Regression coverage includes:

• Benchmark Dashboard
• Trust Assurance
• Privacy Pools
• Compliance Primitives
• fragment preservation

The branch is a single commit and changes only src/lib/render.ts and tests/render-links.test.ts.