Completes the PR-1 migration. The question target now has a single source of truth:
the binding-graph's sole BoundUnknown. The sidecar QuantQuery dataclass + the
QuantComprehension.query field are DELETED.
- New helper single_unknown(graph) -> BoundUnknown | None: returns the sole target, or
None on a graph that does not carry exactly one. Zero unknowns (no question) and
multiple unknowns (ambiguous) both REFUSE — the consumer must never pick one.
- to_relational_metric reads the query from single_unknown(graph) (refuses on None).
- realize_quantitative reads the asked symbol from single_unknown(bg) (NotRealized on None).
- Tests: the .query assertions move to single_unknown; new malformed-graph tests prove
0 and >1 unknowns REFUSE rather than pick one (the wrong=0 boundary).
Byte-identical where it matters: relational_metric answer lane 15/15 wrong=0, setup-oracle
15/15 setup_wrong=0, realize-binding-graph + architectural invariants green. No serving
path touched. No dangling QuantQuery reference remains.
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