Guard against invalid source links

[alandefreitas]

Problem

SourceLocationInfo can point outside the repository root. When that happens we still populate the “path relative to root” field, and the generated pages link to non-existent files.

Proposed solution

• Add a guard in the extraction step: if a location sits outside the configured root, leave the relative-path field blank.
• Audit existing call sites to ensure they respect the empty field and do not try to build a link anyway.
• Add a test case covering an out-of-tree include so the guard stays in place.

Requirements

• Locations outside the root no longer produce broken links.
• Tests cover both in-tree and out-of-tree files.
• Documentation (or inline comments) notes the guard.