bug: rrf order on a traversed binding fails on engine v2

[azimafroozeh]

What happened

With set engine = v2;, a query that orders by rrf(nearest(...), bm25(...)) on a binding reached through a traversal fails with RRF arm has no ranking column 'd._distance'; expected {"d.slug":"d1"}, which the same query returns under set engine = v1;. Found by running .gqt cases on v2 and comparing each step's rows with v1; Steps is the shrunk case in the .gqt corpus format.

Steps to reproduce

--- schema
node Doc {
    slug: String @key
    body: String @index
    embedding: Vector(2) @index
}
node Topic {
    name: String @key
}
edge About: Doc -> Topic

--- seed
{"type":"Doc","data":{"slug":"d1","body":"needle","embedding":[1.0,0.0]}}
{"type":"Doc","data":{"slug":"d2","body":"filler","embedding":[0.0,1.0]}}
{"type":"Topic","data":{"name":"storage"}}
{"edge":"About","from":"d1","to":"storage"}
{"edge":"About","from":"d2","to":"storage"}

--- query
set engine = v2;
query ranked_destination() {
    match { $t: Topic { name: "storage" } $d about $t }
    return { $d.slug }
    order { rrf(nearest($d.embedding, [1.0, 0.0]), bm25($d.body, "needle")) }
    limit 1
}

Version

omnigraph 0.11.0, PR 760 branch planner-m3-read-path-plan @ 09f1956 (engine v2 is not on main yet), built from source

Environment

macOS (Darwin 25.5) arm64, local filesystem store (the .gqt harness runs the embedded engine). Not tried on an object-store backend.

Logs / output

query failed: RRF arm has no ranking column 'd._distance'

[ragnorc]

Retested on main (80406fee), where v2 is on main and v1 is still the default engine.

The reported error is gone. Since #769 the planner refuses the query with a typed error: {"kind":"unsupported_query","message":"\nearest()` orders `$d`, a traversal destination, which engine v2 does not support; …"}`. So v2 no longer fails at execution, but it still does not answer the query that v1 answers.

Variants on the issue's fixture:

Query	v1	v2
rrf(nearest, bm25), $t declared first (issue)	d1	unsupported_query
nearest() only, $t first	error: search-ordered query produced rows without its 'd._distance' ranking column	unsupported_query
bm25() only, $t first	error: … without its 'd._score' ranking column	unsupported_query
rrf(...), $d: Doc declared first	d1	d1
nearest() only, $d first	d1	d1

The single-arm v1 failures are #750's symptom on the default engine; I've reopened #750 for them. Declaring the ranked variable first works on both engines.