fix(build): keep .py sources + make smoke test skip uninstalled modules

- compile_and_strip_sources: stop deleting .py files after compileall.
  OpenCV's loader does literal file I/O on cv2/config.py (not a Python
  import), so stripping it breaks `import cv2` with "missing
  configuration file: ['config.py']". Other packages may do similar
  file-based introspection tricks — the ~30% size win isn't worth
  playing whack-a-mole with broken installers. We already hit this
  with numpy.linalg and zeroconf._services; enough incidents.
- smoke_test_imports: only assert importability for modules whose
  top-level dir actually exists in site-packages. Pillow for example
  is a Windows-only dep, and was failing the Linux build spuriously.
  Rewrote as a heredoc for readability.

build-common.sh

@@ -151,13 +151,20 @@ cleanup_site_packages() {
     echo "  Site-packages after cleanup: $cleaned_size"
 }
 
-# ── Pre-compile .py → .pyc and strip sources ─────────────────
+# ── Pre-compile .py → .pyc (keep sources) ────────────────────
 #
-# MUST run AFTER cleanup_site_packages (so we don't waste work compiling
-# files that are about to be deleted) and BEFORE any step that ships the
-# result. Uses `compileall -b` to produce legacy `foo.pyc` next to
-# `foo.py` (not `__pycache__/foo.cpython-XX.pyc`), which survives the
-# __pycache__ cleanup and works without a matching .py file at import.
+# MUST run AFTER cleanup_site_packages. Speeds up first startup by
+# producing .pyc alongside .py. We deliberately do NOT delete .py
+# sources afterwards:
+#
+#   1. OpenCV's loader does literal file I/O on cv2/config.py (not
+#      an import) — stripping it breaks `import cv2` with:
+#      "OpenCV loader: missing configuration file: ['config.py']".
+#   2. Other packages may do similar tricks (inspect.getsource,
+#      runtime introspection, __file__-relative data loading).
+#   3. The size saving (~30%) isn't worth the whack-a-mole of
+#      shipping broken installers. We already hit this with
+#      numpy.linalg and zeroconf._services — enough incidents.
 #
 # Args:
 #   $1 — directory to compile (site-packages or app/src)
@@ -176,12 +183,9 @@ compile_and_strip_sources() {
         echo "  ERROR: compileall failed for $target_dir — aborting"
         return 1
     }
-
-    echo "  Removing .py source (keeping .pyc)..."
-    # Keep __init__.py so package discovery still works in edge cases
-    # where namespace detection checks for the file.
-    find "$target_dir" -name "*.py" ! -name "__init__.py" -delete 2>/dev/null || true
-    # __pycache__ dirs are redundant now that we have legacy .pyc files
+    # Drop __pycache__ to save the duplicated PEP-3147 copies; the
+    # `-b` flag above placed legacy .pyc next to each .py, so nothing
+    # of value is lost here.
     find "$target_dir" -type d -name __pycache__ -exec rm -rf {} + 2>/dev/null || true
 }
 
@@ -209,32 +213,57 @@ smoke_test_imports() {
         pypath="$extra_path:$sp_dir"
     fi
 
-    # Modules that MUST import cleanly for the app to start.
-    # If you add a new top-level dependency, add it here.
-    PYTHONPATH="$pypath" "$py_cmd" -c "
+    # Modules that MUST import cleanly IF PRESENT. We don't enforce
+    # installation — Pillow for example is only a Windows dep. But if a
+    # module's top-level package dir exists in site-packages and we
+    # can't import it, that's a broken install and we abort.
+    PYTHONPATH="$pypath" "$py_cmd" - "$sp_dir" <<'PYEOF' || {
+        echo "  ERROR: smoke test failed — site-packages is broken, aborting build"
+        return 1
+    }
+import importlib
+import os
 import sys
-modules = [
-    'numpy', 'numpy.linalg', 'numpy.lib', 'numpy.matrixlib',
-    'cv2',
-    'fastapi', 'uvicorn', 'starlette', 'pydantic',
-    'zeroconf',
-    'PIL', 'PIL.Image',
-    'yaml',
+
+sp_dir = sys.argv[1]
+
+# (module_name, site-packages path to check for presence)
+candidates = [
+    ('numpy', 'numpy'),
+    ('numpy.linalg', 'numpy/linalg'),
+    ('numpy.lib', 'numpy/lib'),
+    ('numpy.matrixlib', 'numpy/matrixlib'),
+    ('cv2', 'cv2'),
+    ('fastapi', 'fastapi'),
+    ('uvicorn', 'uvicorn'),
+    ('starlette', 'starlette'),
+    ('pydantic', 'pydantic'),
+    ('zeroconf', 'zeroconf'),
+    ('zeroconf._services', 'zeroconf/_services'),
+    ('PIL', 'PIL'),
+    ('PIL.Image', 'PIL'),
+    ('yaml', 'yaml'),
 ]
+
+tested = 0
+skipped = 0
 failed = []
-for mod in modules:
+for mod, path in candidates:
+    if not os.path.exists(os.path.join(sp_dir, path)):
+        skipped += 1
+        continue
     try:
-        __import__(mod)
+        importlib.import_module(mod)
+        tested += 1
     except Exception as e:
         failed.append(f'{mod}: {type(e).__name__}: {e}')
+
 if failed:
     print('SMOKE TEST FAILED:', file=sys.stderr)
     for f in failed:
         print(f'  {f}', file=sys.stderr)
     sys.exit(1)
-print('  Smoke test passed (imported', len(modules), 'modules)')
-" || {
-        echo "  ERROR: smoke test failed — site-packages is broken, aborting build"
-        return 1
-    }
+
+print(f'  Smoke test passed ({tested} imported, {skipped} not installed)')
+PYEOF
 }