Support repeated GQL node variables with SameAs

[Copilot]

Repeated node variables in GQL patterns must constrain later occurrences to the same graph element. GripQL now provides SameAs for that identity check; repeated edge variables remain explicitly unsupported until edge bindings are available.

• Protocol and engine: Add protobuf field 74, Go and Python builders, and a core filter that compares element IDs. Compilation rejects unknown bindings and vertex/edge type mismatches; Mongo queries fall back to the core engine.
• GQL compiler: Lower repeated node variables to SameAs without overwriting their original binding.
• Coverage and docs: Add serialization, engine, compiler, endpoint, and Python tests; document supported mapping and rejection behavior.

MATCH (n)-[:FRIEND]->(middle)-[:FRIEND]->(n)
RETURN n

gripql.NewQuery().
  V().
  As("n").
  Out("FRIEND").
  As("middle").
  Out("FRIEND").
  SameAs("n").
  Render("$n")

[kellrott]

Rather then leave it as unsupported op, it should be implemented into the mongo pipeline compiler

[Copilot]

Implemented native Mongo pipeline compilation for SameAs, including binding/type validation and mark preservation. Commit: 2e3c30f.

[kellrott]

@copilot can you implement SameAs in the mongo cross compiler, rather then leaving it as an unsupported op?

[Copilot]

@copilot can you implement SameAs in the mongo cross compiler, rather then leaving it as an unsupported op?

SameAs now compiles in Mongo’s aggregation pipeline using $expr to compare current and bound element IDs; vertex/edge and validation tests pass. Commit: 2e3c30f.