feat(sdk): initX402({ maxAmountPerPayment }) to raise the x402 per-payment cap

[Edgadafi]

Closes #106

What

initX402() now accepts an optional maxAmountPerPayment, passed to @x402/core as spendControls.maxAmountPerPayment, so APIs priced above $1 can be paid with Nirium.

Checked against @x402/core 2.28.0 (installed, not older docs)

• x402Client.setSpendControls(controls); SpendControls.maxAmountPerPayment?: Money | false, default DEFAULT_MAX_AMOUNT_PER_PAYMENT = "$1".
• Enforced in applySpendControls, before the payload is created, so a blocked payment never reaches the signer.

Design

• Accepts '$5', '5' or 5, normalized to "$N". Validated at initX402() time: positive amounts only. 0, negatives, NaN, Infinity, exponent/comma forms, null and false throw. Disabling the cap (false) is intentionally not exposed; raising it must be an explicit, bounded choice.
• Without the option setSpendControls is not called, so the $1 default and current behavior are unchanged.
• Applied on both paths: plain x402Fetch and the policy path. With a policy, a payment above the cap is refused before the policy or the signer is consulted.
• X402SpendCapError message now points to the option instead of saying it cannot be changed.

Tests (test/x402-spend-cap-config.test.ts, e2e on dist/ with the in-memory x402-edges fixtures)

• default: a $1.01 payment fails with X402SpendCapError, cap $1, signer never called, no paid retry
• raised ('$2', '2', 2, '1.01'): $1.01 pays, signed exactly once
• raised to $5: just above $5 is still blocked before the signer
• with policy + raised cap: pays under the cap; above it the policy is not asked and the signer is not called
• invalid values throw at init
  Full suite on Node 22.12.0: jest 56/56, node:test 126/126.

Docs

README "Per-payment cap" documents the option and why the $1 default exists. CHANGELOG "Unreleased" entry.

AI disclosure

This PR was written with AI assistance under my direction: the AI read @x402/core 2.28.0, drafted the change, tests and docs. I reviewed the diff and the test results by hand before opening it.