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/expected/1192-diagnostics-many-pointer-open-slice.exit

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

examples/diagnostics/expected/1192-diagnostics-many-pointer-open-slice.stderr

@@ -0,0 +1,5 @@
+error: slicing a many-pointer `[*]T` requires an explicit upper bound (`mp[lo..hi]`) — it has no length
+  --> examples/diagnostics/1192-diagnostics-many-pointer-open-slice.sx:10:10
+   |
+10 |     s := mp[1..];          // ERROR: many-pointer slice needs an explicit hi
+   |          ^^

examples/diagnostics/expected/1192-diagnostics-many-pointer-open-slice.stdout

@@ -0,0 +1 @@
+