fix(wallet): hold reservation on ambiguous x402 settlement (#128)

[VickyXAI]

Summary

• postWithPayment tracks whether the signed (paid) request was dispatched; if the call then aborts / times out, the wallet hold is marked ambiguous instead of being released by the caller's finally.
• WalletReservation keeps ambiguous amounts counted against headroom for a 30s grace window and prunes them on the next fresh on-chain balance read after that window (which already reflects a real settlement). Cap errs tight, never loose; a genuinely-absent spend self-heals.
• Balance fetcher is injectable for tests; new test/reservation.local.mjs (3 tests) wired into npm test.

Closes #128

Test plan

• npm test — 673/673 pass

[VickyXAI]

Follow-up commit after an adversarial pass on the first cut:

• Grace window is now timeoutMs + 30s, not a flat 30s. ModalCreate's own timeout is 90s; if the gateway settles after the paid work finishes, settlement can land up to 90s after our abort, so a flat 30s could prune before the balance read reflected it.
• Provably-unsent failures are not ambiguous: a signal that aborted during signing (before the paid fetch went out) or an ECONNREFUSED/ENOTFOUND on the paid request releases normally.
• Cut-off paid response is ambiguous: previously .text().catch(() => '') turned a 200 with a truncated body into ok: true, body: {}, which for ModalCreate meant a paid sandbox with no id in the tracker. It now throws and holds.
• Prune only runs on a real on-chain read that started after the window closed; the Infinity RPC-failure fallback never prunes.
• Insufficient-funds message names the ambiguous hold instead of "other in-flight calls".
• postWithPayment and the signer are test-injectable; 13 tests cover the ambiguous/not-ambiguous matrix. Suite 683/683.

Not addressed here (pre-existing, flagged for follow-up): the SDK's Solana getBalance() returns 0 on RPC errors rather than throwing, so the "Infinity, don't block on flakiness" fallback never engages on Solana.