[scanner] ChainProvider: alloy WebSocket connect + latest-block health check

[obchain]

Context

All on-chain reads flow through a single ChainProvider abstraction. For v0.1 this manages a WebSocket connection to BNB Chain; the type is designed so multi-chain support in later milestones is a config change, not a rewrite.

Scope

• charon-scanner/src/provider.rs with a ChainProvider struct wrapping an alloy provider
• Constructor takes &ChainConfig from charon-core
• test_connection() -> Result<u64> — fetches and returns the latest block number
• WebSocket connection via ProviderBuilder::new().on_ws(WsConnect::new(url))
• Connection errors surface with chain name + URL in the error context (no panics / no unwraps)

Acceptance criteria

• ChainProvider::new(config) connects over WebSocket
• test_connection() returns the latest block number on a running BSC node
• Connection failure produces a clean error chain with context
• Required alloy feature flags (pubsub, ws, etc.) added to charon-scanner/Cargo.toml

References

• PRD section 5b (Alloy Provider & WebSocket Connections)

[obchain]

Superseded — ChainProvider WS shipped in feat/06-chainprovider-ws.