[GP4-S5] App-Safe Missing Retention Semantics

[flyingrobots]

Roadmap slice: GP4-S5
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,
#517.

Scope:

• Ensure app-safe readings and outcomes report missing retention as typed
  obstruction/posture, not empty success.

Acceptance:

• Missing reading payload bytes return missing-retention posture.
• Missing reading envelope bytes return missing-retention posture.
• Missing receipt evidence does not erase the applied/rejected causal outcome;
  it reports unavailable support material.

Witness:

cargo test -p warp-core retained_reading_missing_payload_is_not_empty_success

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

#319 - [codex] Harden reading artifact posture [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]
#380 - feat(core): expose retained evidence posture on readings [merged]

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