Add immutable revalidated Runtime settlement plans

[flyingrobots]

Parent: #712
Blocked by: Runtime fork and strand Lane slice

Outcome

Runtime.previewSettlement() returns inspectable evidence plus an immutable runtime-backed SettlementPlan; Runtime.settle() accepts only that plan and revalidates all bound frontiers before mutation.

Acceptance

• A plan binds source/target Lane identities, exact frontiers, proposal, law, policy, and plan digests.
• Preview is non-mutating and does not reserve or publish history.
• Plans are frozen, unforgeable at the public boundary, and accepted only by the Runtime that issued them.
• Settle revalidates source and target frontiers and the active law/policy.
• A changed basis produces a stable stale-basis obstruction; it never executes the stale plan.
• Conflict and obstruction cannot settle; lawful plurality may remain plural.
• Settlement receipts use the canonical operation/outcome/evidence vocabulary.
• Real-Git, concurrency, stale-plan, consumer, and boundary tests prove the contract.

Non-goals

Common-basis braid validation and merge-compatible object input are not promised.

[coderabbitai]

🔗 Related PRs

#106 - Advance v18 release-candidate evidence [merged]
#745 - Seal Git substrate out of v19 receipts and evidence [merged]
#766 - Define witnessed admission and settlement contracts [merged]
#768 - Classify v19 writes with admission outcomes [merged]
#769 - feat(api): establish the v19 Runtime and Lane surface [merged]

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