lang 2.3: tuple materialization from a pack — (..xs) / (..xs.method)

A `spread_expr` element inside a tuple literal now expands the pack into the
tuple's fields: `(..xs.get)` ≈ `(xs[0].get(), …, xs[N-1].get())` (Decision 2 —
a pack is stored by materializing a tuple). lowerTupleLiteral detects a
pack-spread element via packSpreadRefs and splices the per-element Refs as
fields (typed via getRefType); for Box(T) the materialized tuple is
heterogeneous. A spread whose operand isn't a pack falls through to the
existing spread_expr diagnostic (tuple-value spread not yet handled).

When any element is a spread, field-count ≠ element-count, so the contextual
target-tuple alignment is skipped (field types inferred from the expanded refs).
examples/198-pack-tuple-materialize.sx.

tests/expected/198-pack-tuple-materialize.exit

@@ -0,0 +1 @@
+0

tests/expected/198-pack-tuple-materialize.txt

@@ -0,0 +1,2 @@
+0=42 1=hi
+0=x 1=7