refactor(8.0): API-surface + quality polish from the readiness audit

- find() search-mode: collapsed the two overlapping options to one canonical
  `searchMode: SearchMode` (the redundant `mode` alias was silently ignored on
  the primary find() path while honored on the historical path — a footgun) and
  removed the unwired `explain?` FindParams field (never read by find()).
- Documentation accuracy on the public type surface: entity ids documented as
  UUID v7 (auto) / v5 (natural key), not v4; dropped the stale "Brainy 3.0"
  banners and "backward compatibility" hedging on the fresh-8.0 Result type;
  documented the via/type alias; removed the dead GraphConstraints.bidirectional;
  fixed the no-op `EmbeddedGraphVerb = Omit<GraphVerb,'source'>` to omit the real
  sourceId; corrected the GraphIndexProvider.isInitialized JSDoc; documented
  removeMany's per-chunk generation granularity; JSDoc'd the public VFS surface.
- Honest perf comments in source: removed unmeasured billion-scale figures from
  the LSMTree / graphAdjacencyIndex headers (the JS fallback is in-memory after
  open; native is the scale path).
- Robustness: getVerbMetadata propagates read errors symmetrically with
  getNounMetadata; the find() egress integrity-guard keeps a row when the JS
  matcher doesn't implement an operator the provider already matched on; hardened
  the boundary-no-native CI guard to catch side-effect imports + re-exports.
- Tests: de-theatricalized a relateMany test to assert the real contract; fixed
  a stale Model-B header.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/graph/graphAdjacencyIndex.ts

@@ -77,9 +77,10 @@ export class GraphAdjacencyIndex implements GraphIndexProvider {
   private lsmTreeVerbsBySource: LSMTree  // sourceId -> verbIds
   private lsmTreeVerbsByTarget: LSMTree  // targetId -> verbIds
 
-  // ID-only tracking for billion-scale memory optimization
-  // Previous: Map<string, GraphVerb> stored full objects (128GB @ 1B verbs)
-  // Now: Set<string> stores only IDs (~100KB @ 1B verbs) = 1,280,000x reduction
+  // ID-only membership tracking: a Set<string> of verb ids rather than a
+  // Map<string, GraphVerb> of full objects, so per-verb resident memory is one id
+  // string instead of a whole relationship record. (Unmeasured order-of-magnitude
+  // figures dropped — the durable property is "ids, not objects".)
   private verbIdSet = new Set<string>()
 
   // Verb-id interning for the BigInt boundary (8.0 u64 contract).