Struct defaults apply inside structs; exit() ends an or-block; 1e9 literals; x * 2.0 stays f32; floats print as the shortest exact text; str(x, digits)

Also frees main's args and the message of a failing main (valgrind was not clean on two tests).

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

tests/new/fail/after_exit.fch

@@ -0,0 +1,5 @@
+// expect: this code can never run (it comes after return, break, continue or exit)
+fn main() {
+    exit(1)
+    print("x")
+}