fix: is-operator, try with generic Result, Unwrap exits on panic

Desugar `is` to tag/value equality in bootstrap and selfhost so LIR
no longer drops hIs as false. Resolve monomorphized Result/Option type
names for `?` (_Tag/_Data). Call bux_exit(1) after Unwrap panic messages.
Add is_operator example and document follow-up fixes.

bootstrap/c_backend.nim

@@ -346,7 +346,8 @@ proc emitExpr(be: var CBackend, node: HirNode): string =
     return &"(({typ}){operand})"
 
   of hIs:
-    return "true"  # TODO: proper type checking
+    # Should be desugared in hir_lower; keep false if any residual hIs remains
+    return "false"
 
   of hSizeOf:
     let typ = typeToC(be, node.sizeOfType)