feat(8.0): thread commit generation through the graph-write provider contract

Graph time-travel needs an edge's existence recorded per generation so
db.asOf(g) hops resolve historically correct endpoints. The metadata layer
already threads brainy's commit generation per write; the graph write path did
not, leaving a versioned verb-endpoint store unable to answer "which edges
existed at generation g".

- GraphIndexProvider.addVerb/removeVerb gain a `generation: bigint` parameter
  (the same watermark the storage layer stamps onto the record). A provider with
  a per-generation edge chain stamps the edge at that generation; the JS baseline
  has no such chain and accepts-and-ignores it — graph time-travel is a
  native-provider capability, and the open-core path serves edges as-of-now (the
  one documented graph time-travel limitation).
- The two graph transaction operations resolve the generation via a thunk at
  EXECUTE time: the generation store assigns the batch generation only once the
  commit begins executing, after the operations are planned. The same generation
  is reused for an operation's rollback half.
- All graph-write call sites pass the in-flight generation.

Adds a spy-provider test proving the threading, execute-time resolution, and
forward/rollback generation reuse. The JS index ignores the value, so behaviour
is unchanged: unit 1402/1402, db-mvcc 25/25, bigint-contract relate/unrelate 10/10.

src/brainy.ts

@@ -283,6 +283,17 @@ export class Brainy<T = any> implements BrainyInterface<T> {
    * `compactHistory()` and the `Db` value type.
    */
   private generationStore!: GenerationStore
+  /**
+   * Resolves the in-flight write generation as a BigInt, for stamping graph
+   * edges via the {@link GraphIndexProvider} contract. Passed to every
+   * `AddToGraphIndexOperation`/`RemoveFromGraphIndexOperation` and evaluated
+   * when the operation executes — inside a `transact()` batch the generation
+   * store has assigned the batch generation by then; for single-op writes it
+   * reads the post-write watermark. The arrow body reads `generationStore`
+   * lazily, so it is safe to define before `init()` assigns the store.
+   */
+  private readonly graphWriteGeneration = (): bigint =>
+    BigInt(this.generationStore.generation())
   /** Lazily built host surface shared by every `Db` value of this brain. */
   private _dbHost?: DbHost<T>
   /**
@@ -2188,7 +2199,7 @@ export class Brainy<T = any> implements BrainyInterface<T> {
           // rollback can re-add through the BigInt addVerb contract)
           const { sourceInt, targetInt } = this.resolveVerbEndpointInts(verb)
           tx.addOperation(
-            new RemoveFromGraphIndexOperation(this.graphIndex, verb, sourceInt, targetInt)
+            new RemoveFromGraphIndexOperation(this.graphIndex, verb, sourceInt, targetInt, this.graphWriteGeneration)
           )
           // Delete verb metadata
           tx.addOperation(
@@ -2580,6 +2591,7 @@ export class Brainy<T = any> implements BrainyInterface<T> {
         tx.addOperation(
           new AddToGraphIndexOperation(
             this.graphIndex, verb, sourceInt, targetInt,
+            this.graphWriteGeneration,
             (verbInt) => this.cacheVerbInt(verbInt, id)
           )
         )
@@ -2618,6 +2630,7 @@ export class Brainy<T = any> implements BrainyInterface<T> {
           tx.addOperation(
             new AddToGraphIndexOperation(
               this.graphIndex, reverseVerb, targetInt, sourceInt,
+              this.graphWriteGeneration,
               (verbInt) => this.cacheVerbInt(verbInt, reverseId)
             )
           )
@@ -2660,7 +2673,7 @@ export class Brainy<T = any> implements BrainyInterface<T> {
         if (verb && endpointInts) {
           tx.addOperation(
             new RemoveFromGraphIndexOperation(
-              this.graphIndex, verb, endpointInts.sourceInt, endpointInts.targetInt
+              this.graphIndex, verb, endpointInts.sourceInt, endpointInts.targetInt, this.graphWriteGeneration
             )
           )
         }
@@ -2785,12 +2798,13 @@ export class Brainy<T = any> implements BrainyInterface<T> {
       if (typeChanged && reindexInts) {
         tx.addOperation(
           new RemoveFromGraphIndexOperation(
-            this.graphIndex, existing, reindexInts.sourceInt, reindexInts.targetInt
+            this.graphIndex, existing, reindexInts.sourceInt, reindexInts.targetInt, this.graphWriteGeneration
           )
         )
         tx.addOperation(
           new AddToGraphIndexOperation(
             this.graphIndex, verbForIndex, reindexInts.sourceInt, reindexInts.targetInt,
+            this.graphWriteGeneration,
             (verbInt) => this.cacheVerbInt(verbInt, params.id)
           )
         )
@@ -4391,7 +4405,7 @@ export class Brainy<T = any> implements BrainyInterface<T> {
               for (const verb of allVerbs) {
                 const { sourceInt, targetInt } = this.resolveVerbEndpointInts(verb)
                 tx.addOperation(
-                  new RemoveFromGraphIndexOperation(this.graphIndex, verb, sourceInt, targetInt)
+                  new RemoveFromGraphIndexOperation(this.graphIndex, verb, sourceInt, targetInt, this.graphWriteGeneration)
                 )
                 tx.addOperation(
                   new DeleteVerbMetadataOperation(this.storage, verb.id)
@@ -5977,7 +5991,7 @@ export class Brainy<T = any> implements BrainyInterface<T> {
     for (const verb of cascade.values()) {
       const { sourceInt, targetInt } = this.resolveVerbEndpointInts(verb)
       plan.operations.push(
-        new RemoveFromGraphIndexOperation(this.graphIndex, verb, sourceInt, targetInt),
+        new RemoveFromGraphIndexOperation(this.graphIndex, verb, sourceInt, targetInt, this.graphWriteGeneration),
         new DeleteVerbMetadataOperation(this.storage, verb.id)
       )
       plan.touchedVerbs.push(verb.id)
@@ -6103,7 +6117,7 @@ export class Brainy<T = any> implements BrainyInterface<T> {
         targetId: params.to
       }),
       new SaveVerbMetadataOperation(this.storage, id, verbMetadata),
-      new AddToGraphIndexOperation(this.graphIndex, verb, sourceInt, targetInt, (verbInt) =>
+      new AddToGraphIndexOperation(this.graphIndex, verb, sourceInt, targetInt, this.graphWriteGeneration, (verbInt) =>
         this.cacheVerbInt(verbInt, id)
       )
     )
@@ -6131,7 +6145,7 @@ export class Brainy<T = any> implements BrainyInterface<T> {
           targetId: params.from
         }),
         new SaveVerbMetadataOperation(this.storage, reverseId, verbMetadata),
-        new AddToGraphIndexOperation(this.graphIndex, reverseVerb, targetInt, sourceInt, (verbInt) =>
+        new AddToGraphIndexOperation(this.graphIndex, reverseVerb, targetInt, sourceInt, this.graphWriteGeneration, (verbInt) =>
           this.cacheVerbInt(verbInt, reverseId)
         )
       )
@@ -6161,7 +6175,7 @@ export class Brainy<T = any> implements BrainyInterface<T> {
     if (verb) {
       const { sourceInt, targetInt } = this.resolveVerbEndpointInts(verb)
       plan.operations.push(
-        new RemoveFromGraphIndexOperation(this.graphIndex, verb, sourceInt, targetInt)
+        new RemoveFromGraphIndexOperation(this.graphIndex, verb, sourceInt, targetInt, this.graphWriteGeneration)
       )
     }
     plan.operations.push(new DeleteVerbMetadataOperation(this.storage, id))