step2.1: rename 64-bit images .MCD -> .MC4 per spec sections 8 and 14

M2c.atg

@@ -13,7 +13,7 @@ COMPILER M2c
      CARDINAL and subranges form one integer family; no mixed
      INTEGER/REAL arithmetic; INTEGER assigns to REAL; 1-character
      literal is CHAR; InvalidType suppresses follow-on errors)
-   - backend (MGen): emits a single-module MC64 image <ModName>.MCD
+   - backend (MGen): emits a single-module MC64 image <ModName>.MC4
      (current directory), runnable with mcint. Lowered: scalar
      globals, CONST literals (startup stores), SET OF ordinals
      (bit masks) with literals, IN, = and #; full scalar

MGen.def

@@ -1,7 +1,7 @@
 DEFINITION MODULE MGen;
 (* MC64 image emitter for the M2c scalar subset (see M2c.atg).
 
-   Buffers a single-module MC64 image and writes "<Module>.MCD"
+   Buffers a single-module MC64 image and writes "<Module>.MC4"
    (current directory) at EndModule. The image satisfies the
    reference loader (Loader2): magic "MC64", descriptor with name,
    TOINIT flag, global sizes (one 8-byte slot per variable), two
@@ -44,7 +44,7 @@ PROCEDURE BeginBody;
 
 PROCEDURE EndModule;
 (* Emits the epilogue, the print procedure, the procedure table,
-   the descriptor, the checksum, and writes the .MCD file. *)
+   the descriptor, the checksum, and writes the .MC4 file. *)
 
 PROCEDURE LoadVar (name: ARRAY OF CHAR);
 (* Pushes the global (placeholder push-0 if unknown). *)

MGen.mod

@@ -814,7 +814,7 @@ PROCEDURE EndModule;
     StrCpy(fname, modName);
     i := StrLen(fname);
     fname[i] := "."; fname[i + 1] := "M"; fname[i + 2] := "C";
-    fname[i + 3] := "D"; fname[i + 4] := 0C;
+    fname[i + 3] := "4"; fname[i + 4] := 0C;
     FileIO.Open(f, fname, TRUE);
     IF FileIO.Okay THEN
       i := 0;

run_tests.sh

@@ -3,7 +3,7 @@
 # Step-2 test runner for M2c (scalars + MC64 backend).
 # Usage: ./run_tests.sh   (from the m2c directory, after ./build.sh)
 #
-# Positive tests: compile with M2c, run the newest .MCD under mcint,
+# Positive tests: compile with M2c, run the newest .MC4 under mcint,
 # compare the printed ExitCode value with the expectation.
 #
 
@@ -52,7 +52,7 @@ expect_run() {
   name="$1"
   want="$2"
   mod=$(head -n 1 "tests/$name" | cut -d' ' -f2 | cut -d';' -f1)
-  mcd="$mod.MCD"
+  mcd="$mod.MC4"
   rm -f "$mcd"
   out=$(./M2c "tests/$name" 2>&1)
   case "$out" in
@@ -79,7 +79,7 @@ expect_run_none() {
   # $1 = test file — must compile and run with empty output
   name="$1"
   mod=$(head -n 1 "tests/$name" | cut -d' ' -f2 | cut -d';' -f1)
-  mcd="$mod.MCD"
+  mcd="$mod.MC4"
   rm -f "$mcd"
   out=$(./M2c "tests/$name" 2>&1)
   case "$out" in