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.