fix: allow null txHash in SettleResult for already-settled retries

[badjer]

Summary

Loosens SettleResult.txHash from string to string | null, adds an optional alreadySettled flag, and updates the two log interpolations that reference txHash to render <already-settled> when it's null.

Why

A companion auth PR (circuitandchisel/auth#new) adds a carve-out on POST /settle/x402 that returns 200 OK with { alreadySettled: true, txHash: null, ... } when the CDP facilitator reports duplicate transaction on a retry of an already-settled payload. Today SettleResult.txHash is typed as non-nullable string, which would be a lie the moment that auth change ships.

Nothing in the SDK reads txHash beyond two logger.info template-string interpolations (protocol.ts:233, atxpExpress.ts:153), so runtime behavior is unchanged on the happy path. This PR just tells the truth at the type layer and makes the log line render nicely when it's null.

What changed

• packages/atxp-server/src/protocol.ts — SettleResult.txHash: string → string | null, add optional alreadySettled?: boolean, and update log interpolation to ${result.txHash ?? '<already-settled>'}.
• packages/atxp-express/src/atxpExpress.ts — same log-interpolation nil-coalesce on the middleware's txHash=... log line.
• packages/atxp-server/src/protocol.test.ts — one new test: auth returns { txHash: null, alreadySettled: true, ... }, SDK surfaces it as-is without crashing.

Ordering / deploy safety

• SDK-first deploy (this PR first, auth second): SDK types allow null but auth never returns null yet — safe.
• Auth-first deploy (auth PR first, SDK still on 0.11.8): auth returns txHash: null, SDK's old type casts as SettleResult — JS runtime is fine (${null} renders as "null", ugly but no crash), the only consumers are log lines. Also safe.
• Either order works; no coordinated release needed.

Test plan

• npx vitest run src/protocol.test.ts (atxp-server) — 26 passed (25 original + 1 new)
• npx vitest run (atxp-express) — 45 passed
• Typecheck clean in main checkout for both packages
• Lint clean on changed files
• Full CI green

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