[XY-1078] Add privacy delete export source retention boundaries

[yvette-carlisle]

Summary

• Add privacy/delete/export/source-retention enforcement across Source Library, Knowledge Workspace, graph/search/readback, recall debug, and benchmark/report docs.
• Add explicit Source Library delete path (DELETE /v2/docs/{doc_id} and elf_docs_delete) that soft-deletes docs and enqueues doc-vector DELETE cleanup.
• Preserve current-read suppression for deleted/private/excluded spans, including proposal-backed Knowledge Workspace citations and recall-debug readback.

Recovery note

• Decodex produced the lane and repeated review-repair commits, then dropped the active lease with a dirty orphaned worktree and no PR.
• Main thread manually validated and committed the final Source Library delete-boundary repair as e11c14b.

Verification

• cargo make fmt
• cargo make check (docs check, cargo check, clippy, vstyle, nextest: 353 passed / 92 skipped)
• Re-ran cargo make check after manual NotFound/delete-scope adjustment: 353 passed / 92 skipped

Authority: XY-1078