fix: slicing a many-pointer yields a correct slice (issue 0159)

emitSubslice handled a struct (slice/string) base and an array base, but a
many-pointer [*]T base is an LLVM pointer kind — it fell through to the else arm
that mapped the result to LLVMGetUndef(slice_ty), so a slice of a many-pointer
(mp[lo..hi]) had a garbage .len/.ptr and iterating it segfaulted.

Add a LLVMPointerTypeKind branch: the base value IS the data pointer, so GEP by
lo and len = hi - lo (the caller supplies the bound; no length is read from the
unbounded pointer). An open-ended mp[lo..] has no resolvable upper bound (a [*]T
carries no length), so lowerSliceExpr now diagnoses it instead of emitting a
.length op that yields garbage.

A List (whose items is [*]T) is now iterable with for items[0..len] (e);
applied in Scheduler.deinit. Regressions: examples/types/0195 (valid slice +
List for-each) + examples/diagnostics/1192 (open-ended rejection).

examples/types/0195-types-many-pointer-slice.sx

@@ -0,0 +1,26 @@
+// Slicing a many-pointer `mp[lo..hi]` builds a correct `{ ptr = mp + lo,
+// len = hi - lo }` slice — the caller supplies the bounds (a `[*]T` carries no
+// length of its own). This makes a `List` (whose `items` is `[*]T`) iterable
+// with a `for`-each over `items[0..len]`.
+//
+// Regression (issue 0159): a many-pointer base previously fell through the
+// subslice emitter's `else` arm to an undefined slice (`LLVMGetUndef`), so the
+// resulting `.len` was garbage and iterating it segfaulted.
+#import "modules/std.sx";
+
+main :: () -> i64 {
+    a : [4]i64 = .[5, 6, 7, 8];
+
+    // Slice a many-pointer with explicit bounds.
+    mp : [*]i64 = xx @a[0];
+    s := mp[1..4];                       // { &a[1], len 3 }
+    print("mp[1..4]: len={} [{} {} {}]\n", s.len, s[0], s[1], s[2]);  // 3 [6 7 8]
+
+    // The payoff: iterate a List with a for-each over items[0..len].
+    xs : List(i64) = .{};
+    xs.append(10); xs.append(20); xs.append(30);
+    sum := 0;
+    for xs.items[0..xs.len] (e) { sum = sum + e; }
+    print("List for-each sum={}\n", sum);                            // 60
+    return 0;
+}

examples/types/expected/0195-types-many-pointer-slice.exit

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

examples/types/expected/0195-types-many-pointer-slice.stderr

@@ -0,0 +1 @@
+

examples/types/expected/0195-types-many-pointer-slice.stdout

@@ -0,0 +1,2 @@
+mp[1..4]: len=3 [6 7 8]
+List for-each sum=60