[Routing] Stabilize route report schema and goldens

[cssbruno]

Source: split follow-up from #237 after the #61 routing/layout/view baseline.

Scope:

• Stabilize gowdk routes schema beyond the current versioned JSON baseline.
• Decide and document generated output path coverage in route reports.
• Clarify route priority wording for static, dynamic, rest, SSR, API, action, fragment, and contract endpoint records.
• Fill source-span completeness gaps for route and endpoint report fields.
• Pin any newly stable fields with deterministic goldens.

Acceptance criteria:

• docs/reference/routing.md documents any newly stable route report fields and clearly labels any remaining planned fields.
• cmd/gowdk/testdata/routes_golden/ pins the stable JSON output.
• Focused tests prove deterministic ordering and source-span shape for the added fields.
• Product rules remain unchanged: explicit .gwdk routes are route truth, and endpoint metadata stays separate from page render modes.

Completed by #318.

Related: #61, #237.