GQ: optional traversals that preserve unmatched root rows

[ragnorc]

Problem

An ordinary traversal removes roots with no matching neighbor. There is no optional traversal construct to retain those roots and return nullable neighbor/edge bindings. Clients retrieving an entity with optional context need separate queries or client-side joins.

An attempted optional { ... } block fails parsing on 0.10.0; this is a requested language capability, not an existing syntax contract being violated.

Proposed change

Define optional match/traversal semantics with nullable bindings and clear variable scope. A root that matches the required portion should survive when the optional pattern has no match. Specify multiple matches, filters inside/outside the optional block, chained optional hops and interaction with aggregation/order/limit.

Acceptance criteria

• Root rows with zero, one and multiple optional matches have documented multiplicity.
• Nullability is reflected in compile-time result types/descriptors and JSON output.
• Optional edge bindings expose properties only when the edge exists; parallel edges retain multiplicity.
• Filters do not accidentally convert an optional traversal into a required one unless explicitly requested.
• Indexed and scan execution produce the same rows, including empty graphs and self-loops.

Scope this issue to optional traversal. Nested maps/collections can be designed separately rather than making them prerequisites. Grammar, type checking, executor.