feat: add single-stage underwriting hook

[tsubasakong]

Summary

• add an experimental single-stage UnderwritingHook that commit-locks underwriting terms on setBudget(...)
• add UnderwritingEvaluator for registered-underwriter EIP-712 complete/reject relays after evidence submission
• document the hook as a Profile C example without the parent/close workflow scaffold

Why

• this extracts the more generic hook piece from #16: a single-stage underwriter-gated completion flow that fits the existing ACP lifecycle
• it intentionally omits the parent/close linkage, coordinator, and larger workflow scaffold so the hook reads as a smaller lego block

Explicitly out of scope

• parent/close linkage
• active close slot management
• AwaitingClose state
• UnderwritingCoordinator
• workflow-only hook surface such as setWiring(...), markProtected(...), jobSettlementJobId(...), isAwaitingClose(...), getParentJobId(...), and getActiveCloseJobId(...)

Workflow-heavy follow-up

• the remaining two-stage parent/close workflow scaffold is intended for the separate public companion repo: https://github.com/tsubasakong/underwriting-settlement-contracts
• that follow-up should carry the workflow-specific coordinator/orchestration code and any larger sequence documentation, rather than expanding hook-contracts

Validation

• forge build --force
• local temporary flow validation covered:
  • single-stage root flow completes with a valid underwriter signature
  • submit rejects when evidence terms do not match the committed underwriting terms

[tsubasakong]

Follow-up note: the workflow-heavy two-stage underwriting scaffold now lives in the public companion repo here:
https://github.com/tsubasakong/underwriting-settlement-contracts

That repo now contains the self-contained workflow package (TwoStageUnderwritingHook / WorkflowCore / Evaluator / Coordinator), workflow-focused docs, and Foundry tests.

So the intended split is now concrete:

• #24 stays the foundational BaseACPHook / setBudget routing fix
• #25 stays the smaller single-stage underwriting hook surface in hook-contracts
• the larger parent/close settlement workflow continues in the companion repo above