docs: add shared build logic pattern and expanded size optimization

- Section 4.3: expanded with NumPy submodule removal, zeroconf cleanup,
  debug symbol stripping, .py source removal, dependency replacement tip
- Section 5.2: new "Shared Build Logic" documenting build-common.sh
  pattern for deduplicating Windows/Linux build scripts

gitea-python-ci-cd.md

@@ -281,19 +281,36 @@ done
 Strip unnecessary files from site-packages to reduce archive size:
 
 ```bash
+# Generic cleanup
 find "$SITE_PACKAGES" -type d -name __pycache__ -exec rm -rf {} +
 find "$SITE_PACKAGES" -type d -name tests -exec rm -rf {} +
 find "$SITE_PACKAGES" -type d -name "*.dist-info" -exec rm -rf {} +
 find "$SITE_PACKAGES" -name "*.pyi" -delete
 
 # Remove build-time-only packages
-rm -rf "$SITE_PACKAGES"/{pip,setuptools,pkg_resources}*
+rm -rf "$SITE_PACKAGES"/{pip,setuptools,pkg_resources,_distutils_hack}*
 
-# Remove heavy unused parts of specific libraries
-rm -f "$SITE_PACKAGES"/cv2/opencv_videoio_ffmpeg*.dll  # -28 MB
-rm -rf "$SITE_PACKAGES"/numpy/{tests,f2py,typing}
+# OpenCV: remove ffmpeg DLL (~28 MB), Haar cascades, dev files
+rm -f "$SITE_PACKAGES"/cv2/opencv_videoio_ffmpeg*.dll
+rm -rf "$SITE_PACKAGES"/cv2/{data,gapi,misc,utils,typing_stubs,typing}
+
+# NumPy: remove unused submodules (only keep core, fft, random)
+for mod in polynomial linalg ma lib distutils f2py typing _pyinstaller; do
+    rm -rf "$SITE_PACKAGES/numpy/$mod"
+done
+
+# zeroconf: remove service type database (~1-2 MB)
+rm -rf "$SITE_PACKAGES/zeroconf/_services"
+
+# Strip debug symbols from native extensions
+find "$SITE_PACKAGES" -name "*.pyd" -exec strip --strip-debug {} \;
+
+# Remove .py source files (keep compiled .pyc only)
+find "$SITE_PACKAGES" -name "*.py" ! -name "__init__.py" -delete
 ```
 
+**Tip:** If a library is only needed for one feature (e.g., Pillow for system tray icons), move it to an optional dependency group and strip unused plugins. Replace its core usage with a library already in the dependency tree (e.g., use cv2 for JPEG encoding instead of Pillow).
+
 ### 4.4. Bundling tkinter (Optional)
 
 Embedded Python doesn't include tkinter. Extract it from the official MSI packages:
@@ -351,6 +368,47 @@ Include install/uninstall scripts for running as a service:
 # Then: systemctl daemon-reload && systemctl enable && systemctl start
 ```
 
+### 5.2. Shared Build Logic
+
+When building for both Windows and Linux, extract shared logic into a `build-common.sh` sourced by both platform scripts. This avoids duplicating version detection, site-packages cleanup, frontend builds, and app file copying.
+
+```bash
+# build-common.sh — shared functions
+detect_version() { ... }      # git tag → env var → pyproject.toml fallback
+clean_dist() { ... }          # rm -rf + mkdir
+build_frontend() { ... }     # npm ci && npm run build
+copy_app_files() { ... }     # cp src/, config/, clean .map/__pycache__
+
+# Parameterized by platform — extension suffix differs:
+#   cleanup_site_packages <path> <ext_suffix> <lib_suffix>
+#   Windows: cleanup_site_packages "$SP" "pyd" "dll"
+#   Linux:   cleanup_site_packages "$SP" "so"  "so"
+cleanup_site_packages() {
+    local sp_dir="$1" ext_suffix="${2:-so}" lib_suffix="${3:-so}"
+    # Generic: __pycache__, tests/, *.dist-info, *.pyi
+    # NumPy: remove unused submodules (polynomial, linalg, ma, etc.)
+    # OpenCV: remove ffmpeg, Haar cascades, dev files
+    # Pillow: remove unused image format plugins
+    # zeroconf: remove _services/ database
+    # strip --strip-debug on *.$ext_suffix
+    # Remove .py source (keep .pyc only)
+}
+```
+
+```bash
+# build-dist-windows.sh / build-dist.sh
+source "$(dirname "$0")/build-common.sh"
+detect_version "${1:-}"
+clean_dist
+# ... platform-specific Python setup + dependency install ...
+cleanup_site_packages "$SITE_PACKAGES" "pyd" "dll"  # or "so" "so"
+build_frontend
+copy_app_files
+# ... platform-specific launcher + packaging ...
+```
+
+**Key principle:** Both scripts run on Linux (Windows build is cross-compiled). Keep platform-specific code (embedded Python, ._pth patching, .bat launchers, systemd services) in the platform scripts. Keep size optimization, cleanup, and build steps that are identical in the common file.
+
 ## 6. NSIS Installer (Windows)
 
 Cross-compilable on Linux: `apt install nsis && makensis installer.nsi`