sync: clamp suggested scan ranges to the current chain tip in initialize

[pacu]

Fixes #636.

Summary

initialize's catch-up loop busy-looped at full speed — pinning a core and writing ~40 log lines/second indefinitely — whenever suggest_scan_ranges returned a range starting above the current chain view's tip. scan_blocks has nothing to scan for such a range and returns Ok without breaking, and the Ok(_) arm re-ran the loop instantly with no state change, so nothing inside the loop could ever resolve the condition (as reported: Scanning blocks Historic(3413618..3414133) against Latest block height is 3413617, repeating indefinitely).

The wallet database can legitimately be ahead of the chain view: it records the highest tip it has ever observed, while Zebra's read-only secondary serves finalized on-disk state that can lag (or effectively regress across a restart).

Fix

Clamp every suggested range to the current tip when selecting the next range to scan (select_initial_scan_range, extracted for testability):

• a range straddling the tip scans its available prefix — real progress each iteration;
• a range entirely above the tip is dropped, and when nothing scannable remains the existing no-range arm exits initialize to steady_state, which waits for chain-tip changes properly instead of polling.

This also covers the silent variant of the same spin where tree_state_as_of(start - 1) returns None (range starting more than one block above the tip), which previously re-looped without even logging.

Verification

• 7 new unit tests on select_initial_scan_range, including the reported repro (range 3413618..3414133 against tip 3413617 → dropped), straddle clamping, Verify-vs-Historic boundary handling, and above-tip ranges not masking later scannable candidates. cargo test -p zallet-core: 201 passed.
• End-to-end regression: wallet_z_shieldcoinbase.py (fresh-wallet initialize + 100-block catch-up scan) passes against a release build of this branch on the zebrad+zainod regtest stack.

🤖 Generated with Claude Code

https://claude.ai/code/session_017WB1TU6tyLmMa1n4SQ9XgK