fix: optional-chain index opt?.xs[i] over array/ptr-array field (issue 0181)

opt?.xs[i] typed and lowered the index over the optional CONTAINER
(?[N]T); getElementType returned .unresolved, so index_get reached LLVM
with an unresolved element type and panicked. Mirroring the 0101
!-unwrap fix: add lowerOptionalChainIndex (optional_has_value -> some:
unwrap + index (index_gep+load for ?*[N]T, else index_get) +
optional_wrap; none: const_null; merge -> ?ElemType, element-optional
flattened). The typer + dispatch guard compute the element via
ptrToArrayElem(child) orelse getElementType(child), so value-arrays,
slices, many-pointers, AND pointer-to-array (?*[N]T) children resolve.
Null receivers short-circuit (no null deref).

Regression: examples/optionals/0915-optional-chain-array-field-index.sx.
Verified by 3 adversarial reviews, suite 794/0. Filed broader pre-existing
gap 0183 (indexing a non-indexable type panics instead of diagnosing).

src/ir/lower/expr.zig

@@ -1021,6 +1021,49 @@ pub fn lowerOptionalChain(self: *Lowering, obj: Ref, fa: *const ast.FieldAccess,
     return self.builder.blockParam(merge_bb, 0, result_ty);
 }
 
+/// Lower an indexed optional-chain access: `opt?.xs[i]` where the `?.` field is
+/// an array / slice / many-pointer. Mirrors `lowerOptionalChain`'s short-circuit
+/// — the index applies in the some-branch, producing `?ElemType` (null when the
+/// receiver was null). `child` is the unwrapped container type, `elem_ty` the
+/// indexed element type.
+pub fn lowerOptionalChainIndex(self: *Lowering, ie: *const ast.IndexExpr, child: TypeId, elem_ty: TypeId) Ref {
+    // The chained `?.` field access produced the optional value; lower it.
+    const opt_val = self.lowerExpr(ie.object);
+    // If the element is itself optional, indexing flattens (no double-wrap),
+    // matching the field-chain `?.` flattening rule.
+    const elem_is_optional = !elem_ty.isBuiltin() and self.module.types.get(elem_ty) == .optional;
+    const result_ty = if (elem_is_optional) elem_ty else self.module.types.optionalOf(elem_ty);
+
+    const has_val = self.builder.emit(.{ .optional_has_value = .{ .operand = opt_val } }, .bool);
+
+    const some_bb = self.freshBlock("chain.some");
+    const none_bb = self.freshBlock("chain.none");
+    const merge_bb = self.freshBlockWithParams("chain.merge", &.{result_ty});
+
+    self.builder.condBr(has_val, some_bb, &.{}, none_bb, &.{});
+
+    // Some: unwrap the container, index it. A `?*[N]T` unwraps to the pointer
+    // (GEP through it); a value container (`?[N]T` / `?[]T`) unwraps to the
+    // aggregate value and `index_get`s the element.
+    self.builder.switchToBlock(some_bb);
+    const unwrapped = self.builder.emit(.{ .optional_unwrap = .{ .operand = opt_val } }, child);
+    const idx = self.lowerExpr(ie.index);
+    const elem_val = if (self.ptrToArrayElem(child)) |pelem| blk: {
+        const gep = self.builder.emit(.{ .index_gep = .{ .lhs = unwrapped, .rhs = idx } }, self.module.types.ptrTo(pelem));
+        break :blk self.builder.load(gep, pelem);
+    } else self.builder.emit(.{ .index_get = .{ .lhs = unwrapped, .rhs = idx } }, elem_ty);
+    const some_result = if (elem_is_optional) elem_val else self.builder.emit(.{ .optional_wrap = .{ .operand = elem_val } }, result_ty);
+    self.builder.br(merge_bb, &.{some_result});
+
+    // None: null optional.
+    self.builder.switchToBlock(none_bb);
+    const none_result = self.builder.constNull(result_ty);
+    self.builder.br(merge_bb, &.{none_result});
+
+    self.builder.switchToBlock(merge_bb);
+    return self.builder.blockParam(merge_bb, 0, result_ty);
+}
+
 /// Field access on a known type (shared by regular field access and optional chaining)
 /// Map a Vector swizzle component (`.x`/`.y`/`.z`/`.w` or the colour
 /// aliases `.r`/`.g`/`.b`/`.a`) to its lane index. Returns null for any
@@ -1755,6 +1798,24 @@ pub fn lowerIndexExpr(self: *Lowering, ie: *const ast.IndexExpr) Ref {
     }
     // Infer element type from the object's slice/array type
     const obj_ty = self.inferExprType(ie.object);
+    // Optional-chain index: `opt?.xs[i]`. The `?.` makes the object an
+    // optional whose child is the (array/slice/many-ptr) field — so the index
+    // applies inside the chain's some-branch and the whole expression is
+    // `?ElemType` (null if the receiver was null). Without this the element
+    // type resolved through `getElementType(?[N]T)` was `.unresolved` and an
+    // `index_get` on the optional value reached LLVM emission (issue 0181).
+    if (!obj_ty.isBuiltin() and self.module.types.get(obj_ty) == .optional) {
+        const child = self.module.types.get(obj_ty).optional.child;
+        // A pointer-to-array child (`?*[N]T`) is indexable too: its element is
+        // the pointee array's element. `getElementType` has no pointer arm, so
+        // ask `ptrToArrayElem` first (mirrors the non-optional `*[N]T` path
+        // below) — otherwise the `?*[N]T` case fell through to a plain
+        // `index_get` with an `.unresolved` element type (issue 0181).
+        const elem_ty = self.ptrToArrayElem(child) orelse self.getElementType(child);
+        if (elem_ty != .unresolved) {
+            return self.lowerOptionalChainIndex(ie, child, elem_ty);
+        }
+    }
     // Array with addressable storage: GEP the element in place + load,
     // never `index_get` on the loaded array VALUE — that realizes as
     // copy-whole-array-to-temp per read (the general-expression sibling