[GP3-S4] CAS-Addressed WSC Export Fixture

[flyingrobots]

Roadmap slice: GP3-S4
Goalpost: Goalpost 3: WSC Causal-History Export And Import
Parent: #522
Related: #521

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

Scope

Parent issues:
#522,
#519.

Scope:

• Export WSC graph facts plus content-addressed refs to WAL segments and
  retained material.
• Use echo-cas as byte storage, not semantic authority.

Acceptance:

• Import succeeds when required CAS blobs are present and match their content
  hashes.
• Import reports missing CAS blobs as typed missing material.
• Equal bytes under different semantic coordinates do not alias.

Witness:

cargo test -p warp-core wsc_cas_addressed_export_requires_present_blobs
cargo test -p echo-cas semantic_retention

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

#263 - feat(echo-cas): content-addressed blob store (Phase 1) [merged]
#298 - feat(warp-core): TTD domain logic — provenance storage + causal cone walk [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]

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