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/diagnostics/1192-diagnostics-many-pointer-open-slice.sx

@@ -0,0 +1,12 @@
+// Slicing a many-pointer `[*]T` requires an explicit upper bound — it carries
+// no length, so an open-ended `mp[lo..]` has no bound to resolve and would
+// otherwise build a garbage-length slice. This guards that diagnostic.
+// (Companion: examples/types/0195 covers the valid explicit-bound form.)
+#import "modules/std.sx";
+
+main :: () -> i64 {
+    a : [4]i64 = .[5, 6, 7, 8];
+    mp : [*]i64 = xx @a[0];
+    s := mp[1..];          // ERROR: many-pointer slice needs an explicit hi
+    return s.len;
+}