fix(ir): missing struct field assignment errors cleanly, no LLVM panic [F0.10]

Assigning to a nonexistent struct field (`p.q = 2` where Point has no `q`)
aborted the compiler with the `.unresolved` LLVM tripwire instead of a source
diagnostic (issue 0094). The lvalue field lookup never diagnosed a miss:

- `lowerAssignment`'s `.field_access` target left `field_ty = .unresolved` when
  no struct field matched, then built `ptrTo(field_ty)` and stored — so a
  pointer-to-`.unresolved` reached LLVM emission and tripped the panic.
- `lowerExprAsPtr`'s `.field_access` fallback returned
  `structGepTyped(obj_ptr, 0, .s64, obj_ty)` on a miss — a silent field-0/`.s64`
  default that mislowered the lvalue.

Both sites now reuse the read path's `emitFieldError` (the exact facility
`lowerFieldAccessOnType` uses), so read and write reject identically with
`field 'q' not found on type 'Point'`. `lowerExprAsPtr` also resolves
union/tagged-union fields via `union_gep` (the old `.s64` fallback was silently
standing in for union field access — e.g. `u.a[0] = v`), so that path is fixed,
not just made loud. The `types.zig` tripwire is untouched: the fix is to never
produce `.unresolved` for a missing-field store.

Regression tests:
- examples/1145-diagnostics-missing-struct-field-assign.sx — negative, both
  sites error, exit 1.
- examples/0165-types-nested-struct-field-assign.sx — positive, nested struct
  field write + address-of a matched field still work.
- src/ir/lower.test.zig — lowering unit test asserting the field-not-found
  diagnostic for a missing-field assignment.

examples/expected/0165-types-nested-struct-field-assign.exit

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

examples/expected/0165-types-nested-struct-field-assign.stderr

@@ -0,0 +1 @@
+

examples/expected/0165-types-nested-struct-field-assign.stdout

@@ -0,0 +1,2 @@
+a=11 b=22 tag=33
+a2=111

examples/expected/1145-diagnostics-missing-struct-field-assign.exit

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

examples/expected/1145-diagnostics-missing-struct-field-assign.stderr

@@ -0,0 +1,11 @@
+error: field 'q' not found on type 'Point'
+  --> examples/1145-diagnostics-missing-struct-field-assign.sx:18:5
+   |
+18 |     p.q = 2;                              // site 1: lowerAssignment target path
+   |     ^^^
+
+error: field 'missing' not found on type 'Outer'
+  --> examples/1145-diagnostics-missing-struct-field-assign.sx:21:5
+   |
+21 |     o.missing.a = 5;                      // site 2: lowerExprAsPtr fallback
+   |     ^^^^^^^^^

examples/expected/1145-diagnostics-missing-struct-field-assign.stdout

@@ -0,0 +1 @@
+