Completes the issue-0094 fix. attempt-1 made single-assign and address-of
diagnose a missing struct field; the stress-review found two remaining defects
in that change:
1. lowerMultiAssign's `.field_access` target kept the pre-fix shape — a
struct-only loop that defaulted `field_idx 0` / `field_ty .unresolved` on a
miss, then built the GEP and stored unconditionally. A missing field
(`p.q, y = 2, 3`) silently wrote field 0 (printed `x=2 y=3`, no diagnostic),
and a valid promoted-union / tuple member at a non-zero offset corrupted
field 0 instead of its own slot.
2. attempt-1's new union branch in lowerExprAsPtr resolved only DIRECT union
field names, so `@v.x` on a promoted anonymous-struct member reported
"field 'x' not found on type 'Vec2'" even though `v.x = 41` worked.
Both lvalue-pointer sites and the multi-assign store now route through one
shared resolver, `fieldLvaluePtr`, that handles struct fields, union direct
fields, promoted anonymous-struct union members, and tuple elements, and
returns null (no field-0 / `.unresolved` default) on a genuine miss. Each
caller emits the read path's `emitFieldError` on null. This collapses the
three previously-divergent field-lvalue walks into one, fixing the
multi-assign missing-field corruption, the promoted-member over-rejection,
and (as a side effect of correct resolution) non-zero-offset promoted-union
and tuple multi-assign stores. The types.zig tripwire is untouched.
Regression tests:
- examples/1145 extended: multi-assign missing field (`p.r, y`) errors, exit 1.
- examples/0166 (new): promoted union member written and address-of'd,
including a non-zero-offset member (`@v.y`), compiles and runs.
- src/ir/lower.test.zig: multi-assign missing-field field-not-found unit test.
