feat: add amalgamation generation script and Makefile target for wren.c

Introduce a Python script that concatenates all Wren library source files into a single amalgamation file, resolving local `#include` directives recursively to eliminate internal header dependencies. Add a corresponding `wren.c` Makefile target that invokes the script on the core header and VM sources, along with `.DELETE_ON_ERROR` to clean up on failure.

script/generate_amalgamation.py

@@ -0,0 +1,28 @@
+#!/usr/bin/env python
+
+import sys
+import os.path
+import re
+
+INCLUDE_PATTERN = re.compile(r'^\s*#include "([\w.]+)"')
+
+seen_files = set()
+
+def add_file(filename):
+  basename = os.path.basename(filename)
+  # Only include each file at most once.
+  if basename in seen_files:
+    return
+  seen_files.add(basename)
+  path = os.path.dirname(filename)
+
+  with open(filename, 'r') as f:
+    for line in f:
+      m = INCLUDE_PATTERN.match(line)
+      if m:
+        add_file(os.path.join(path, m.group(1)))
+      else:
+        sys.stdout.write(line)
+
+for f in sys.argv[1:]:
+  add_file(f)