fix(0117): pointer-to-array indexing auto-derefs

A '*[N]T' receiver in an index expression reached LLVM emission with an
unresolved element type and tripped the panic sentinel — no read or
write spelling worked. ptrToArrayElem on Lowering recognises the shape;
the index READ path GEPs the pointee array through the pointer value
and loads the element; the write / compound-assign / lvalue /
addr-of-element paths and the expression typer resolve the element type
through the same helper (their GEP machinery already handled a pointer
base). Kept out of getElementType so slice paths don't half-accept a
raw pointer base.

Regression: examples/0176 (read, write, compound, element ptr + deref).

examples/0176-types-pointer-to-array-index.sx

@@ -0,0 +1,21 @@
+// Indexing through a pointer-to-array auto-derefs: `p : *[N]T` makes
+// `p[i]` GEP the pointee array and load the element, and `p[i] = v` /
+// `p[i] += v` store through it — mirroring the struct-pointer auto-deref
+// on field access. Writes through the pointer are visible in the
+// original array and vice versa.
+//
+// Regression (issue 0117): this shape used to reach LLVM emission with
+// an unresolved element type and panic.
+
+#import "modules/std.sx";
+
+main :: () {
+    k : [4]s64 = .[11, 22, 33, 44];
+    p := @k;
+    print("read={}\n", p[2]);
+    p[1] = 99;
+    p[3] += 1;
+    print("write={} {} {}\n", k[1], p[1], k[3]);
+    e := @p[2];
+    print("elem-ptr={}\n", e.*);
+}

examples/expected/0176-types-pointer-to-array-index.exit

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

examples/expected/0176-types-pointer-to-array-index.stderr

@@ -0,0 +1 @@
+

examples/expected/0176-types-pointer-to-array-index.stdout

@@ -0,0 +1,3 @@
+read=33
+write=99 99 45
+elem-ptr=33