fix: diagnose indexing a non-indexable type instead of panicking (issue 0183)

lowerIndexExpr fell through to an index_get with an .unresolved element
type for any non-indexable object (*T, *[]T, struct, scalar), reaching
codegen -> 'unresolved type reached LLVM emission' panic. Add a guard
after all indexable arms: if getElementType(obj_ty) is .unresolved and
obj_ty is itself resolved (genuinely non-indexable, not a prior-error
placeholder), emit a located 'cannot index a value of type <T>'
diagnostic + placeholder (hasErrors aborts before codegen). A single
pointer hints by pointee: ptr-to-scalar -> many-pointer/dereference;
ptr-to-array/slice -> dereference first. No false-positives (generics,
aliases, late-resolved, every indexable shape verified).

Regression: examples/diagnostics/1203-diagnostics-index-non-indexable.sx.
Verified by 3 adversarial reviews, suite 799/0. Filed adjacent pre-existing
panic 0184 (untyped positional .{ } literal with no target type).

examples/diagnostics/expected/1203-diagnostics-index-non-indexable.exit

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

examples/diagnostics/expected/1203-diagnostics-index-non-indexable.stderr

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+error: cannot index a value of type '*i64' — use a many-pointer '[*]T', or dereference first
+  --> examples/diagnostics/1203-diagnostics-index-non-indexable.sx:23:19
+   |
+23 |     print("{}\n", p[0]);    // error: '*i64' is not indexable (hint form)
+   |                   ^
+
+error: cannot index a value of type 'S'
+  --> examples/diagnostics/1203-diagnostics-index-non-indexable.sx:26:19
+   |
+26 |     print("{}\n", s[0]);    // error: 'S' is not indexable (bare form)
+   |                   ^

examples/diagnostics/expected/1203-diagnostics-index-non-indexable.stdout

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+