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/types/graphTypes.ts

@@ -399,10 +399,12 @@ export interface GraphVerb {
 }
 
 /**
- * Version of GraphVerb for embedded relationships
- * Used when the source is implicit from the parent document
+ * A {@link GraphVerb} for relationships embedded under a parent noun, where the
+ * source is implicit (the parent), so `sourceId` is dropped. (`GraphVerb` has no
+ * `source` field — the prior `Omit<GraphVerb, 'source'>` was a no-op that left the
+ * shape identical to `GraphVerb`; this omits the real `sourceId`.)
  */
-export type EmbeddedGraphVerb = Omit<GraphVerb, 'source'>
+export type EmbeddedGraphVerb = Omit<GraphVerb, 'sourceId'>
 
 // Proper Noun interfaces - extend GraphNoun with specific noun types