feat(host-reth): replace ExEx backfill with direct DB reads

[prestwich]

Summary

• Replace ExEx-driven backfill (which re-executes blocks) with direct DB reads from the reth provider, fixing slow startup and memory issues
• New DbBackfill<P> reads blocks+receipts in batches up to finalized, then hands off to the ExEx stream for live blocks
• HostChain enum unifies backfill and live chain segments behind Extractable
• set_head documented and enforced as once-only across both notifier implementations

Details

The ExEx backfill mechanism re-executes historical blocks on startup, which is slow and has caused memory issues. The reth DB already contains executed results, making re-execution unnecessary.

New types (signet-host-reth):

• DbBlock / DbChainSegment — owned block+receipts from DB, implements Extractable
• DbBackfill<P> — batch reader using spawn_blocking for MDBX reads
• HostChain — enum wrapping DbChainSegment (backfill) and RethChain (live)

Notifier changes:

• RethHostNotifier::next_notification is now two-phase: drain DB backfill, then switch to ExEx
• set_head creates a DbBackfill instead of calling ExEx set_with_head
• ExEx stream is initiated after backfill completes, pointed at the last backfilled block
• reth-stages-types dependency removed

Cross-crate:

• HostNotifier::set_head trait doc clarified as once-only
• RpcHostNotifier::set_head guards against repeated calls

Closes ENG-1784

Test plan

• cargo clippy passes (both --all-features and --no-default-features)
• RUSTDOCFLAGS="-D warnings" cargo doc passes
• All existing tests pass (signet-host-reth, signet-node-types, signet-host-rpc)
• signet-node compiles cleanly with new HostChain type
• Integration test on a reth node with historical data (manual)

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