[GP4-S2] Durable Retained Blob Tier

[flyingrobots]

Roadmap slice: GP4-S2
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,
#512.

Scope:

• Add a filesystem-backed or host-supplied durable tier for echo-cas.
• Preserve content-only hash semantics.

Acceptance:

• Bytes written by content hash survive process reconstruction.
• put_verified rejects mismatched bytes without changing the store.
• Missing blobs remain absence, not generic I/O success.
• Listing, if added, returns sorted BlobHash values.

Witness:

cargo test -p echo-cas disk_tier

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]
#309 - feat: add Doghouse PR flight recorder [merged]
#323 - Witnessed suffix admission evaluator [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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