test: prove --no-checks removes the bounds check

The bounds-no-checks cases emitted and built but never ran, so they only
proved that --no-checks produces compilable C -- not that it removes the
check, which is the entire point of the flag.

A run case could not simply be appended. BOUNDS.FE returns the out-of-bounds
element directly, so with checks removed its exit code is whatever sits past
the array on the stack and there is no correct status to assert. Asserting on
the generated C instead does not work either: emit_c.c defines fe_trap_bounds
unconditionally and --no-checks only suppresses the call sites.

Add NOCHK.FE, which reads one element past a [2]i32 and returns x - x. That
is 0 for whatever garbage the unchecked read produced, so the same source has
a defined outcome both ways: compiled with checks it must trap, compiled with
--no-checks it must run to completion and exit 0. Register both halves and
drop the two BOUNDS-N cases they supersede.

Verified in DOSBox-X: 155 passed, including m3-nochk-trap failing as expected
and m3-nochk-off-run succeeding.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BScg8CF1sAAM2zVHAu5zvW

fec/tests/m3/nochk.fe

@@ -0,0 +1,7 @@
+unit m3_no_checks;
+
+fn main() -> i32 {
+    let a: [2]i32 = [1, 2];
+    let x: i32 = a[2];
+    return x - x;
+}

src/ferrolang_vm/registry.py

@@ -146,10 +146,18 @@ CASES: list[Case] = [
     # These two must trap at runtime: the bounds check is the feature under test.
     *_triple(3, "bounds", M3, run_suffix="trap", run_ok=False),
     *_triple(3, "slcbound", M3, run_suffix="trap", run_ok=False),
-    _case(3, "bounds-no-checks-emit",
+    # --no-checks is proved by a differential on one source. NOCHK.FE reads one
-          _emit(_fe(M3, "bounds"), f"{M3}\\BOUNDS-N.C", flags=("--no-checks",))),
+    # element past a [2]i32 and returns x - x, which is 0 whatever garbage the
-    _case(3, "bounds-no-checks-build",
+    # unchecked read produced: compiled with checks it must trap, compiled with
-          _wcl(f"{M3}\\BOUNDS-N.EXE", f"{M3}\\BOUNDS-N.C")),
+    # --no-checks it must run to completion. BOUNDS.FE cannot serve as the
+    # unchecked half because it returns the out-of-bounds value directly, so its
+    # exit code would be whatever happens to sit past the array on the stack.
+    *_triple(3, "nochk", M3, run_suffix="trap", run_ok=False),
+    _case(3, "nochk-off-emit",
+          _emit(_fe(M3, "nochk"), f"{M3}\\NOCHK-N.C", flags=("--no-checks",))),
+    _case(3, "nochk-off-build",
+          _wcl(f"{M3}\\NOCHK-N.EXE", f"{M3}\\NOCHK-N.C")),
+    _case(3, "nochk-off-run", f"{M3}\\NOCHK-N.EXE"),
     *_rejects(3, M3, ("badfld", "badmat", "badarr", "badcycle", "badstr", "badchar",
                       "badfield", "badindex"), suffix="reject"),