fix(ir): missing-field multi-assign + promoted-union-member lvalue [F0.10]

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.

examples/1145-diagnostics-missing-struct-field-assign.sx

@@ -1,13 +1,15 @@
 // Assigning to a field that does not exist on a struct produces the same
 // `field 'X' not found on type 'Y'` diagnostic as the read path (1100), and
-// exits 1 — never the `.unresolved` LLVM-emission panic.
+// exits 1 — never the `.unresolved` LLVM-emission panic, never a silent store
+// into a neighbouring field.
 //
 // Regression (issue 0094): the lvalue field lookup left `field_ty = .unresolved`
 // (lowerAssignment's assignment-target path) or silently GEP'd field 0 as `.s64`
-// (lowerExprAsPtr's fallback), so a missing-field store built a
-// pointer-to-`.unresolved` that panicked at LLVM emission. Both the
-// assignment-target path (`p.q`) and the nested lvalue-pointer path
-// (`o.missing.a`) now emit the field-not-found diagnostic.
+// (lowerExprAsPtr's fallback / lowerMultiAssign's struct loop), so a missing-field
+// store either built a pointer-to-`.unresolved` that panicked at LLVM emission or
+// silently wrote field 0. All three lvalue sites now emit the field-not-found
+// diagnostic: the assignment-target path (`p.q`), the nested lvalue-pointer path
+// (`o.missing.a`), and the multi-target store path (`p.r, y`).
 
 Point :: struct { x: s64; }
 Inner :: struct { a: s64; }
@@ -20,5 +22,8 @@ main :: () -> s32 {
     o := Outer.{ inner = Inner.{ a = 1 } };
     o.missing.a = 5;                      // site 2: lowerExprAsPtr fallback
 
+    y : s64 = 0;
+    p.r, y = 3, 4;                        // site 3: lowerMultiAssign field path
+
     return 0;
 }