Add immediate Fidget-style DSL

[treeform]

Summary

• Rebase the immediate Fidget-style DSL work on top of current master.
• Add the DSL as silky/fidgetdsl.nim and keep the semantic testing layer restored.
• Move menu code out of widgets.nim into silky/menus.nim, including the existing menu bar/submenu API plus the DSL menuRoot, menuItem, and popupMenu helpers.
• Use the shared subWindow widget in the calculator, basic window, and 7GUIs examples instead of local window templates.
• Restore the example test folders and remove the standalone examples/fidgetdsl examples from the branch, runner, and CI.

Validation

• nim c --hints:off examples/basicwindow/basicwindow.nim
• nim c --hints:off examples/calculator/calculator.nim
• nim c --hints:off examples/menu/menu.nim
• nim c --hints:off examples/the7gui/the7gui.nim
• (cd examples/basicwindow && nim r --hints:off --nimcache:tests/nimcache_basicwindow tests/test.nim)
• (cd examples/calculator && nim r --hints:off --nimcache:tests/nimcache_calculator tests/test.nim)
• (cd examples/gameplayer && nim r --hints:off --nimcache:tests/nimcache_gameplayer tests/test.nim)
• (cd examples/layouts && nim r --hints:off --nimcache:tests/nimcache_layouts tests/test.nim)
• (cd examples/menu && nim r --hints:off --nimcache:tests/nimcache_menu tests/test.nim)
• (cd examples/panels && nim r --hints:off --nimcache:tests/nimcache_panels tests/test.nim)
• (cd examples/the7gui && nim r --hints:off --nimcache:tests/nimcache_the7gui tests/test.nim)
• nim r --hints:off -d:silkyTesting --nimcache:tests/nimcache_semantic tests/test_semantic.nim
• nim r --hints:off --nimcache:tests/nimcache_textboxes tests/test_textboxes.nim
• nim r --hints:off --nimcache:tests/nimcache_atlas tests/test_atlas_png.nim
• git diff --check