Constant DEFAULT_KEYMATCH_TOP_K 

pub const DEFAULT_KEYMATCH_TOP_K: u64 = _; // 512u64

Per-source top-k when a KeyMatch-rooted rule omits max_edges.

Equal to MAX_KEYMATCH_LIST, the most destinations a single KeyMatch source can ever name: a list-valued FK field must fire on every element by default. A scalar FK field names at most one destination, so the higher cap is inert there — the top-k filter has nothing to truncate.