chore: use new order-stream filter from sdk

[Fraser999]

[Claude Code]

Summary

Replaces hand-rolled tx_cache.stream_orders() loops with OrderStreamExt::filter_orders from signet-sdk's new stream-combinators module (ENG-1789).

Changes

• Cargo.toml - bumped signet-{constants,orders,tx-cache,types,zenith} to 0.16.3.
• crates/tracker-server/Cargo.toml - added signet-orders dep so the server can import OrderStreamExt.
• crates/tracker-server/src/ingestion/order_discovery.rs - poll_cache uses filter_orders with a stateful predicate that records every hash into current_hashes and returns true only for newly-seen ones, then try_collects the new orders. Same observable behavior, fewer moving parts.
• crates/tracker-server/src/ws/handlers.rs - single-order WS lookup uses filter_orders + try_next instead of a manual while let with a hash check.
• crates/tracker/src/order_tracker.rs - fetch_order rewritten the same way.
• CLAUDE.md - added signet-orders to the tracker-server dependency list.

[Fraser999]

• chore: use new order-stream filter from sdk #3 👈 (View in Graphite)
• main

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[Evalir]

lgtm, this looks super clean now