[XY-1064] Productize Knowledge Workspace source-backed pages

[yvette-carlisle]

Summary

• Add Source Library document and document-span inputs to Knowledge Workspace rebuilds.
• Add author/timeline page kinds plus source refs, generated_by metadata, version identity, and memory-candidate policy metadata.
• Strengthen API/eval/docs/tests so derived pages remain source-backed projections and do not mutate Memory Ledger directly.

Validation

• cargo make fmt
• cargo make lint-fix
• cargo make checks

Notes

Decodex attempt xy-1064-attempt-1-1782116231 retained partial progress after repo gate rewrote tracked files. Main thread accepted the gate-stable diff, reran the registered gate successfully, and committed with decodex commit --authority XY-1064. This PR is intended for Decodex review-handoff adoption.