feat: replace TxPoller polling with SSE streaming

[Evalir]

Summary

Replaces the 1s timer-based polling loop in TxPoller with SSE streaming for real-time transaction delivery from the tx-pool. The new task lifecycle:

1. Startup: full paginated fetch of all transactions currently in the cache
2. Steady state: SSE stream (/transactions/feed) pushes new transactions as they arrive — no more redundant refetches
3. Block env change: full refetch to ensure consistency (covers any items the SSE stream may have missed)

On SSE disconnect or error, the poller reconnects with exponential backoff (1s initial, doubling up to 30s cap) and does a full refetch to cover the gap. Backoff resets on each successfully received transaction.

Changes

• Cargo.toml: enable sse feature on init4-bin-base (transitively enables signet-tx-cache/sse)
• src/tasks/cache/tx.rs: rewrite TxPoller — replace poll loop with full_fetch() + subscribe() + select! over SSE items and block env changes. Add reconnect() with exponential backoff. Remove poll_interval_ms, poll_duration(), Default impl.
• src/tasks/cache/system.rs: pass block_env watch receiver to TxPoller::new()
• tests/tx_poller_test.rs: update integration test to use TxCache directly (no more check_tx_cache() method)

BundlePoller is unchanged — the /bundles/feed server endpoint is not yet available.

Test plan

• make clippy passes clean
• make test — all 8 unit tests pass, integration tests correctly ignored
• Manual test against tx-pool: verify SSE subscription log, real-time tx delivery, refetch on block env change, reconnect with backoff on disconnect

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