[executor] Private RPC submission for BSC (bloxroute / blocknative) + per-chain routing

[obchain]

Context

Public BSC mempool is front-runnable. Private RPC providers (bloxroute, blocknative) let you submit transactions directly to validators, bypassing the public mempool. Ethereum would use Flashbots instead — this issue covers BSC only; the Ethereum path comes later with multi-chain.

Scope

• Add [submission] section to config: per-chain submission_url + optional auth header
• Submitter in charon-executor/ that routes per chain_id:
  • BSC → private RPC endpoint (user-configured)
  • Future chains → Flashbots bundle / private sequencer (stubs for now)
• Auth via HTTP Bearer header (most private RPCs use tokens)
• Fallback to public endpoint with WARN log if private endpoint fails

Acceptance criteria

• Config schema supports per-chain submission URL + optional auth
• Submitter::submit(tx) selects the right endpoint for each chain
• Private endpoint failure falls back to public RPC with a WARN log (and metric)
• Works end-to-end against a test bloxroute account (can be skipped in CI)

References

• PRD section 6a (Flashbots & Private Mempools)
• bloxroute BSC: https://docs.bloxroute.com/bsc

[obchain]

Superseded — private-RPC submitter shipped in feat/19-private-rpc-submit.