Fix the two known limits: adding map keys while looping over the map is a runtime error; temporaries held by an expression are freed when try / or-blocks leave early. err usable after or, inner err shadows outer.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/fail/map_add_while_looping.fch

@@ -0,0 +1,7 @@
+// expect: a key was added to the map while looping over it
+fn main() {
+    m := ["a": 1]
+    for k in m {
+        m[k + "!"] = 2
+    }
+}