[contracts] fork tests rely on BNB_HTTP_URL which publicnode cannot fulfill — archive node required

[obchain]

Refs #53

File: contracts/foundry.toml, [rpc_endpoints] section; contracts/test/CharonLiquidatorFork.t.sol

Problem:
foundry.toml maps the 'bnb' fork alias to ${BNB_HTTP_URL}. The repo documentation and PR #52 both reference publicnode (https://bsc-dataseed.publicnode.com) as the default RPC. publicnode is a full node, not an archive node. Fork tests that read historical state (vToken exchange rates, borrower balances at a pinned block) require archive-depth eth_call. publicnode returns 'missing trie node' for these calls.

This was confirmed in issue #229 raised during PR #52 review. PR #53 introduces a new consumer of the same BNB_HTTP_URL alias without resolving the underlying RPC requirement.

Impact:
CI environments using publicnode as BNB_HTTP_URL will see intermittent 'missing trie node' failures or silently incorrect state reads. The '4/4 pass' result in the PR description is only valid against the specific archive RPC the author used locally.

Fix:

1. Add a CI README note (or .env.example comment) specifying that BNB_HTTP_URL must point to a BSC archive node. Recommended: QuickNode BSC archive, Alchemy BSC, or self-hosted.
2. Add a test_fork_realContractsHaveCode-style guard that also checks block.number matches the pinned FORK_BLOCK, so mismatched RPC configuration fails loudly.
3. Resolve issue [scanner] MempoolMonitor::run returns anyhow::Result on public library API — use thiserror #229 by documenting the archive requirement in README.md under the 'Running fork tests' section.

[obchain]

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