fix(maple): handle broken poolCollaterals, add PoR divergence check

[spalen0]

Summary

Maple's poolCollaterals GraphQL resolver currently fails on unregistered native assets (PT_sUSDE, PT_USR), causing a recurring warning. This PR stops fetching the broken field and replaces the lost monitoring with:

• Proof-of-Reserves divergence check — cross-checks proofOfReserves.totalCollateralValue against syrupGlobals.collateralValue and alerts at ≥0.1% divergence.
• Unknown collateral asset detection — uses collateralDisclosure { asset } to alert on newly disclosed assets not in ASSET_RISK_SCORES.
• Errors routed to dedicated errors channel — GraphQL skip/failure messages now go to the errors topic instead of the Maple alert group.
• README + risk scores updated for the currently disclosed collateral set.

Changes

• protocols/maple/collateral.py:
  • Remove poolCollaterals from the GraphQL query; rename fetch_collateral_data() to fetch_pools_data().
  • Remove per-asset weighted risk score calculation (no working data source).
  • Add fetch_proof_of_reserves() and check_proof_of_reserves().
  • Add fetch_collateral_disclosure() and check_unknown_collateral_assets().
  • Update ASSET_RISK_SCORES with newly disclosed assets.
• protocols/maple/README.md:
  • Document PoR divergence and unknown-asset checks.
  • Remove obsolete per-asset risk score line; fix collateralization ratio threshold to 135%.