relaymonitor: page forward-fill through dense windows

[damilolaedwards]

Summary

Forward-fill fetched one page per tick and advanced the cursor to the highest slot in the response. On a relay with many builder submissions per slot, a single limit-capped page can return only the top few slots of the requested range, so the cursor jumped straight to the top of the window while the slots below it were never fetched. Because forward-fill only ever moves the cursor forward, those slots were skipped for good.

fetchForwardFillWindow now pages down through the window, lowering the cursor to just below the lowest slot seen each time the response comes back truncated, until the window is fully covered down to the current position or a page budget is hit. The cursor is only advanced once the whole window is proven covered. If the budget runs out on a very dense window, the location is left unchanged and the same window is retried on the next tick rather than silently skipping part of it.

Test plan

• go build ./...
• go test ./pkg/relaymonitor/... -race
• New tests cover a window that pages through cleanly within the budget (asserting every slot is delivered and the cursor lands at the expected value), and a window too dense for the budget (asserting no location update happens rather than a partial, gap creating one)