feat: update PLAN.md and add end to end test of the compiler

e2e.sh

@@ -0,0 +1,56 @@
+#!/bin/bash
+
+# Exit immediately if a command exits with a non-zero status.
+set -e
+
+# --- Helper Function ---
+run_test() {
+    local src_file="$1"
+    local expected_code="$2"
+    local base_name
+    base_name=$(basename "$src_file" .src)
+    local obj_file="tests/$base_name.o"
+    local exec_file="tests/$base_name"
+
+    echo "--- Running test: $src_file ---"
+
+    # 1. Compile the source file using our compiler
+    echo "  [1/4] Compiling..."
+    cargo run --release -- "$src_file" -o "$obj_file" > /dev/null 2>&1
+
+    # 2. Link the object file with the system linker (gcc)
+    echo "  [2/4] Linking with gcc..."
+    gcc "$obj_file" -o "$exec_file"
+
+    # 3. Run the executable
+    echo "  [3/4] Running..."
+    set +e
+    ./"$exec_file"
+    local actual_code=$?
+    set -e
+
+    # 4. Check the exit code
+    echo "  [4/4] Verifying exit code..."
+    if [ "$actual_code" -eq "$expected_code" ]; then
+        echo "SUCCESS: Exit code is $actual_code as expected."
+    else
+        echo "FAILURE: Expected exit code $expected_code, but got $actual_code."
+        exit 1
+    fi
+
+    # Clean up generated files
+    rm "$obj_file" "$exec_file"
+    echo "------------------------------------"
+    echo
+}
+
+# --- Test Cases ---
+
+# Test a simple positive return value.
+run_test "tests/return_42.src" 42
+
+# Test a negative return value. Shell exit codes are unsigned 8-bit integers,
+# so -69 (i8) wraps around to 187.
+run_test "tests/return_neg_69.src" 187
+
+echo "All end-to-end tests passed!"