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/exit_or.fch

@@ -0,0 +1,12 @@
+// exit(...) ends an or-block, like return
+fn load(str p) -> int! {
+    return error("no such file")
+}
+
+fn main() {
+    n := load("a.txt") or {
+        print("stopping:", err)
+        exit(0)
+    }
+    print("never", n)
+}