Verbatim relocation of the 19-method coercion cluster (lowerXX, user
conversions, protocol erasure, default-value construction, zero values,
coerceToType implicit/explicit ladder, C-variadic promotion, call-arg
coercion) plus the nested single-home CoerceMode enum into
src/ir/lower/coerce.zig. 19 aliases on Lowering keep all call sites
unchanged.
Method pub-flip: prependCtxIfNeeded. ParamImplEntry stays a Lowering
nested type (field type of param_impl_map) and is reached via an alias
const.
Gate: zig build OK; zig build test 426/426; run_examples 541/0; zero
expected/ snapshot churn.
