[GP4-S4] Retained Evidence WSC Export

[flyingrobots]

Roadmap slice: GP4-S4
Goalpost: Goalpost 4: Retained Evidence Durability
Parent: #519
Related: #512, #513, #517, #522

Source: docs/design/wal-wsc-durability-roadmap.md on branch cycle/521-wal-wsc-storage-relationship at bde7957c.

Scope

Parent issues:
#519,
#522.

Scope:

• Export recovered retained material and reading refs through WSC.
• Support ref-only, self-contained, and CAS-addressed retained evidence modes.

Acceptance:

• Ref-only export names semantic coordinate and material digest without
  claiming bytes are available.
• Self-contained export can reveal the retained payload after import.
• CAS-addressed export succeeds only when required content hashes are present.

Witness:

cargo test -p warp-core wsc_retained_evidence_export_modes

Dedupe note

This is the PR-sized implementation slice for the roadmap item above. Existing umbrella or adjacent issues remain linked as Parent/Related rather than duplicated here.

[coderabbitai]

🔗 Related PRs

#323 - Witnessed suffix admission evaluator [merged]
#329 - Stack Witness 0001 fixture-backed jedit walking skeleton [merged]
#371 - feat(core): advance v0.1 contract evidence and retention [merged]
#379 - feat(core): commit runtime WAL ACK evidence [merged]
#551 - Finish braid historical membership and replay goalpost [merged]

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