refactor(B7.2): move pack/variadic lowering to lower/pack.zig
Verbatim relocation of the 19-method pack cluster (pack element/value
projection, spread refs, pack indexing + diagnostics, pack-to-slice,
variadic arg lowering/packing, pack-slice materialisation, pack-fn
calls + monomorphisation, pack projection resolution, isPackFn/
isPackParam predicates) into src/ir/lower/pack.zig. 19 aliases on
Lowering keep all call sites unchanged.
ProjectionPosition and PackProjection move from lower/protocol.zig to
lower/pack.zig (their domain home, next to resolvePackProjection);
the Lowering type aliases are repointed, so external references
(lower.test.zig) are unchanged.
Method pub-flip: findReturnValueType.
Gate: zig build OK; zig build test 426/426; run_examples 541/0; zero
expected/ snapshot churn.
6b07346d98