fix: harden Arc Testnet settlement

[SuPuHe]

Summary

Harden Arc Testnet settlement and readiness checks while leaving Arc Mainnet unchanged and out of scope until its planned launch.

Changes

• Use pending nonces, bigint-safe hex quantities, gas estimation, and a per-provider submission lock for Privy raw fallback.
• Collapse same-intent duplicate raw submissions and conservatively retain ambiguous raw-send failures.
• Verify live USDC decimals and Arc Testnet gas-wallet balance/headroom during production readiness.
• Add readiness, concurrency, duplicate-delivery, and ambiguous-failure regression tests.

Validation

• PASS: pnpm.cmd test (64 files, 966 tests)
• PASS: pnpm.cmd typecheck
• PASS: pnpm.cmd lint
• PASS: pnpm.cmd format:check
• PASS: pnpm.cmd check:generated
• PASS: focused privy-adapter, arc-adapter, and testkit-settlement tests
• PASS: pnpm.cmd test:integration completed; 12 DB-backed tests skipped because no local container runtime was available.

Gate A

• Base SHA: 95709a8
• Candidate tree: f71cb383a1adcf5672a7767da9f9e788f9829463
• Commit: 25a3596
• Reviewer: free-pi-cli / deepseek-v4-flash
• Verdict: VERDICT: PASS

Residual risks

• The in-process fallback lock/map protects one provider instance; the durable OneShot idempotency gate remains the authority across workers and restarts.
• Gate B will record the exact remote head and CI results.

Human merge

Please review and merge manually after Gate B passes.