fix(malachite): gate proposal parts by validator peers

[ukint-vs]

Summary

• Replaces the Malachite validator public-key table with validator identities: address -> { public_key, peer_id }.
• Validates configured Malachite peer IDs at load/startup, including malformed IDs, duplicates, missing active validator identities, and local validator peer-id mismatches.
• Builds and refreshes the active validator peer allowlist, then drops proposal parts from non-validator peers before stream buffering or pending DB writes.
• Updates the example TOML files to document the validator-identities JSON shape.

Split out from #5514 so the bounded proposal-validation PR stays focused.

Tests

• rtk rg "validator-pub-keys" ethexe/.ethexe.example.toml ethexe/.ethexe.example.local.toml
• rtk cargo nextest run -p ethexe-cli validator_identities
• rtk cargo nextest run -p ethexe-malachite-core
• rtk cargo nextest run -p ethexe-malachite
• rtk cargo clippy -p ethexe-cli -p ethexe-malachite-core -p ethexe-malachite --all-targets -- -D warnings
• rtk git diff --check origin/master...HEAD

[gemini-code-assist]

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