fix(sync): return 0 for empty history_min_height to fix validator sync lag

[qj0r9j0vc2]

Summary

• Fix validator sync lag issue where one of four validators consistently falls behind during devnet startup
• Changed get_history_min_height() to return 0 instead of 1 when history is empty

Problem

At startup, tip_height=0 but history_min_height=1 was returned, creating an empty range 1..=0. Peers couldn't serve any height from this invalid range, causing lagging validators to fail sync.

Solution

Return history_min_height=0 when empty, creating valid range 0..=0. Peers correctly report no blocks available until first block is decided.

Test plan

• Added unit tests for empty history case
• All 56 consensus tests pass
• Full workspace tests pass

[qj0r9j0vc2]

Added second fix: set queue_capacity = 10 in engine config.

Root cause discovered: The BoundedQueue for buffering consensus inputs had capacity 0 (default), causing all sync responses for future heights to be silently dropped. This prevented validators from catching up via sync.