Adds a swappable sort:topK seam for the find() result-ranking hot path. Brainy
ranks candidate results by relevance score then slices to offset+limit; for large
candidate sets that is an O(N log N) full sort even though only the page is kept.
New utils/resultRanking.ts exposes rankIndicesByScore (top-K index selection) with a
JS default (sortTopKIndicesJs) that is byte-identical in ordering to the previous
results.sort((a, b) => b.score - a.score) + slice: descending by score, stable ties
by original index (ES2019+ stable sort semantics). setSortTopKImplementation lets a
native provider (e.g. cortex's Rust partial-sort) replace it; the provider returns
indices into a scores array and only has to match the JS index ordering.
brainy.ts resolves the sort:topK provider in init() (same pattern as distance:sq8)
and wires both relevance-score sort sites in find() through rankIndicesByScore +
reorderByIndices. rankIndicesByScore validates a native provider's output (length,
range, no duplicates) and falls back to JS on any inconsistency, so a query never
returns wrong or duplicated results. No provider registered = unchanged JS behavior.
Tests: unit coverage of the dispatcher (JS ordering vs Array.sort across 200 random
trials incl. ties, provider routing, and fallback on bad/throwing providers) plus
find() integration proving ranking routes through a registered provider, that the
provider and JS fallback produce identical ids/scores on the same data, and that
pagination requests k = offset + limit with descending ordering.
