fix(dist): stop stripping .py sources; wipe payload on NSIS upgrade

Two root causes for the 'imaging extension was built for another version
of Pillow' error users hit after install:

1) cleanup_site_packages ran 'find ... -name "*.py" ! -name "__init__.py"
   -delete' with a comment claiming 'keep .pyc only' — but no compileall
   step exists. Result: the dist shipped __init__.py + .pyd only, missing
   every submodule (Image.py, ImageDraw.py, _version.py, ...). Fresh
   installs were broken; in-place upgrades produced a half-old/half-new
   site-packages. Removed the deletion entirely.

2) NSIS installer extracted over the previous install without cleaning
   python/, app/, scripts/. Upgrades left stale files (old PIL/_version.py
   next to new PIL/_imaging.pyd) which raised the Pillow ABI mismatch.
   Wipe those subtrees before File /r, preserving config.yaml at the
   install root.

build-common.sh

@@ -109,6 +109,13 @@ cleanup_site_packages() {
     # Strip debug symbols from native extensions
     find "$sp_dir" -name "*.$ext_suffix" -exec strip --strip-debug {} \; 2>/dev/null || true
 
-    # Remove .py source files (keep .pyc only) — saves ~30-40% on pure-Python packages
-    find "$sp_dir" -name "*.py" ! -name "__init__.py" -delete 2>/dev/null || true
+    # NOTE: do NOT strip .py source files. A previous version of this function
+    # ran `find ... -name "*.py" ! -name "__init__.py" -delete` with a comment
+    # claiming "keep .pyc only" — but no compileall step exists, so the dist
+    # shipped with __init__.py + .pyd only, missing every submodule (Image.py,
+    # ImageDraw.py, _version.py, ...). Fresh installs would fail with
+    # ModuleNotFoundError; in-place upgrades over an older install produced a
+    # half-old/half-new site-packages where PIL/__init__.py was new but
+    # PIL/_version.py was stale, yielding the runtime "_imaging extension was
+    # built for another version of Pillow" import error.
 }