packaging: macOS app and dmg, Linux tar.gz and AppImage, Windows zip

packaging/macos/package.sh builds FieldCraft.app (io.github.jyviko.fieldcraft, with icon.icns
and the layered Assets.car), signs it with a Developer ID and notarizes it when the
credentials are there and ad-hoc otherwise, and puts it on a drag-to-Applications dmg that
is mounted and checked; fieldcraft-cli goes in its own zip. packaging/linux/package.sh makes
an FHS tar.gz (binaries, .desktop entry, hicolor icons, licences) and an AppImage made with
appimagetool and the type 2 runtime at pinned, hash-checked releases.
packaging/windows/package.ps1 zips fieldcraft.exe and fieldcraft-cli.exe (static CRT,
checked PE headers and icon). Every package carries README.md, LICENSE-MIT, LICENSE-APACHE,
NOTICE and THIRD-PARTY-LICENSES.txt (#72).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

packaging/linux/io.github.jyviko.fieldcraft.desktop

@@ -0,0 +1,13 @@
+[Desktop Entry]
+Type=Application
+Name=FieldCraft
+GenericName=Finite Element Modeler
+Comment=Turn geometry into finite element models: mesh, set up, solve and export decks
+Exec=fieldcraft %F
+TryExec=fieldcraft
+Icon=io.github.jyviko.fieldcraft
+Terminal=false
+StartupNotify=true
+StartupWMClass=io.github.jyviko.fieldcraft
+Categories=Science;Engineering;
+Keywords=cae;fea;fem;finite element;mesh;hex;tet;solver;abaqus;nastran;ls-dyna;