Replace xmrchain view-key detection with local PHP scanning

[SlowBearDigger]

Summary

Closes #131.

This PR replaces the xmrchain dependency in view-key mode with local PHP payment detection against a Monero daemon.

Each WooCommerce order receives a deterministic subaddress. WordPress fetches bounded block windows from monerod and verifies outputs locally using the merchant's private view key.

No block explorer or continuously running monero-wallet-rpc service is required.

Main changes

• Local view-key scanner in PHP
• One subaddress per WooCommerce order
• Multiple-node failover and optional node authentication
• Partial payments, top-ups, overpayments and confirmations
• WooCommerce Blocks and HPOS support
• Restored payment views, admin list, shortcodes and discount support
• Setup validation for address, view key, node and network

Runtime model

Payment scanning is checkpointed and bounded.

• Reconciliation runs every 5 minutes through WP-Cron
• Each run processes at most 8 active orders
• Each order scans a limited block window with a time budget
• An hourly job handles unpaid-order expiry
• The customer payment page can also request a bounded status update

This is not a full-chain rescan on every cron run.

Requirements

• PHP 8.0+
• GMP and BCMath
• A reachable full monerod RPC node matching the address network

Demo

Stagenet demo:

https://live.xmrpay.shop/monerowp/shop/

The demo uses a disposable stagenet wallet and real stagenet transactions.

Review focus

I would especially appreciate feedback on:

• the local scanner architecture
• cron limits and catch-up behavior
• the vendored cryptographic surface
• whether compatibility and parity changes should remain together or be split into follow-up PRs