Make identical external-action settlement retries idempotent

[flyingrobots]

Outcome

An adapter may resubmit the exact previously admitted external-action settlement after an acknowledgement loss. Echo returns the original admitted settlement without appending history or granting effect authority. A different settlement for the same request and attempt obstructs as conflicting.

Evidence

cargo test -p warp-core --test external_action_protocol_tests duplicate_settlement_is_a_recovery_obstruction -- --exact passes on c684ddbb6ebb83edb4658ae590192e1eee83721e because the current protocol classifies an identical duplicate as DuplicateSettlement.

ExternalActionCoordinatorV1::claim_grant correctly refuses to recreate adapter execution authority after settlement. The missing surface is a no-effect reconciliation path for a retained settlement candidate.

Acceptance

• exact retries return the original AdmittedExternalActionSettlementV1 and original settlement commit digest;
• exact retries append no WAL frame or commit;
• differing request, attempt, adapter, kind, schema, basis, canonical result, or evidence obstructs as ConflictingSettlement;
• retry reconciliation cannot return a claim grant or execute an adapter;
• recovered settled history supports the same exact-retry result after restart;
• malformed candidates fail existing schema and digest validation before comparison;
• fixed-seed mutation cases and bounded repeated retries preserve one settlement commit;
• the provider/compiler seam and application vocabulary remain unchanged.

Consumer

Blocks flyingrobots/hello-echo#10.

[coderabbitai]

🔗 Related PRs

#670 - Add trusted causal anchor admission [merged]
#688 - Route executable-operation Actions through scheduler-owned Ticks [merged]
#700 - Expose duplicate state evidence from the generic runner [merged]
#704 - Add durable external-action settlement protocol [merged]
#706 - Host bounded workspace observations [open]

---

📝 Issue Planner

_{Check the box below or use the @coderabbitai plan command to generate an implementation plan and prompts that you can use with your favorite coding assistant.}

• Create Plan

---

🧪 Issue enrichment is currently in open beta.

You can configure auto-planning by selecting labels in the issue_enrichment configuration.

To disable automatic issue enrichment, add the following to your .coderabbit.yaml:

issue_enrichment:
  auto_enrich:
    enabled: false

💬 Have feedback or questions? Drop into our discord!