Add git source docs, weekly FreeCAD, update_check URLs

- FreeCAD updated to weekly-2026.03.19 development builds
  Source: github.com/FreeCAD/FreeCAD/releases (weekly tags)
- dwl package: added update_check URL and git source comments
- README: added standalone repo notice and submodule reference

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

desktop/freecad/freecad.toml

@@ -1,14 +1,23 @@
 [package]
 name = "freecad"
-version = "1.0.0"
-description = "Parametric 3D CAD modeler"
+version = "weekly-2026.03.19"
+description = "Parametric 3D CAD modeler (weekly development build)"
 url = "https://www.freecad.org/"
 license = "LGPL-2.0"
 
 [source]
-url = "https://github.com/FreeCAD/FreeCAD/archive/refs/tags/${version}/FreeCAD-${version}.tar.gz"
+# Weekly builds from FreeCAD GitHub releases
+url = "https://github.com/FreeCAD/FreeCAD/releases/download/${version}/freecad_source_${version}.tar.gz"
 sha256 = "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"
 
+# Alternative: build from git main branch for latest development
+# Uncomment git/branch and set sha256 = "SKIP" to use git source instead
+# git = "https://github.com/FreeCAD/FreeCAD.git"
+# branch = "main"
+
+# Upstream update check — queries GitHub releases for new weekly tags
+update_check = "https://api.github.com/repos/FreeCAD/FreeCAD/releases/latest"
+
 [dependencies]
 run = ["glibc", "python", "qt6-base", "opencascade", "boost", "xerces-c", "freetype", "zlib", "libpng"]
 build = ["gcc", "cmake", "ninja", "pkg-config", "swig"]