Fix/fc 887 query cache - #9

Use string-based context for queries

resolve_join_at_open resolved EVERY GROUP BY key on the terminal dim, so a
fact-column key (#7's `GROUP BY yearNum (date dim), shipMethod (fact)`) failed
scalar_column_for_var(terminal_dim, shipMethod) and declined → 156s materialized
fold → deadline. Route each key to its single source instead: fact-column keys
read inline from the fact scan, dim-attribute keys from the FK→GKey map, composed
per SPARQL order.

- KeySource plan + assemble_group_key interleave fact-inline and dim-resolved
  positions; group_cols stays one-per-position so the output binding() is
  unchanged. Both existing paths are special cases (all-Fact = single-table,
  all-Dim = today's join, byte-identical).
- Gate Q1 (route_group_key_sources): a key's source must be EXACTLY ONE
  participating pattern — 0 (unbound), ≥2 (cross-source value-equality the fold
  cannot enforce), or an interior-dim source all decline (v1 admits fact or the
  terminal dim only). No fact-wins tiebreak.
- NULL in ANY key position drops the fact row (BGP unbound-object semantics):
  assemble_group_key's per-position Null-drop is the robust guarantee, with
  validity_cols (a fact group key is a fact object var) as the secondary drop; dim
  keys drop via the map-miss. Symmetric across sources.
- Q2 plain-literal gate applied per key on ITS OWN source pattern/TM (fact AND
  dim). O1/E2 star_constraints, #1490 dup-key decline, and the memory/cancel
  checkpoints are untouched. Empty-dim-subset (all-fact keys over a join)
  degenerates the FK→GKey map to a join-existence set.
- Hermetics: route_group_key_sources (mixed both orders, cross-source/interior/
  unbound declines, all-fact); assemble_group_key (interleave both orders,
  fact-null + dim-null drops, no-resolver defensive drop, and the empty-dim-subset
  existence-only slice `Some(&[])`).
- Corpus q066 (FACT_SHIPMENT mixed COUNT via FROM: string fact key + integer dim
  key; COUNT-only because SHIP_COST is xsd:double and the f64 SUM is
  summation-order dependent) + q067 (FACT_SUPPORT_TICKET mixed COUNT + SUM over the
  xsd:integer csatScore — exact i128, hash-deterministic — exercising the mixed key
  path together with a value fold). Oracles blessed at the wave-4 corpus gate.

Riders folded per review:
- W4-3 DEFENSIVE cancellation: values.rs ValuesOperator now polls check_cancelled
  per batch, bounding the O(input_rows × value_rows) match for a large VALUES
  product. This is capacity protection, NOT the round-3b #9 fix — #9's timeout is
  the un-lowered full scan (the VALUES→IN-set lowering is the real fix, next-slate);
  this poll only makes the execution-side product cancellable.
- W4-1: pin "-0" (parses to 0, canonical "0" ≠ "-0" → declines) and a >i64
  overflow string (parse Err → declines) in the coercion refutation hermetic.