scaffold Flutter app — kernel, extensions, widgets, Tier 0 built-ins

First real content under app/. Lays the whole Tier 0 foundation in one
commit because the pieces depend on each other circularly (kernel →
extension → widgets → built-ins all reference types from the layer
below); splitting would leave intermediate commits that don't compile.

Key shapes:

  * bare WidgetsApp root — no Material, no Cupertino, no Scaffold.
    ClideTheme InheritedWidget is the only source of tokens.
  * ClideKernel InheritedWidget aggregates 18 services (settings,
    project, extensions, theme, panels, events, ipc, commands +
    palette + keybindings, clipboard, files, notify, dialog, tray,
    secrets, os, net, focus, log, i18n). ExtensionContext exposes
    them through a stable interface.
  * ClideExtension + sealed ContributionPoint hierarchy (Tab,
    StatusItem, Toolbar, Command, TrayItem, LayoutPreset). One
    manifest ships N contributions into kernel slots.
  * Three-tier theme pipeline: palette (named colors) → semantic
    roles → ~60 surface tokens. Defaults at each layer so legacy
    palette-only themes produce a complete SurfaceTokens.
  * A11y baked in from day one — every interactive primitive wraps
    in Semantics(label:, hint:, button:); ensureSemantics() at boot;
    theme/contrast.dart helper exposes token pairs for the WCAG
    gate in the a11y test suite.
  * i18n ported from fframe's L10n pattern (text-driven, namespaced
    JSON catalogs, caller-supplied placeholders) with a proper
    locale fallback chain (exact → language → default → placeholder)
    fframe lacks.
  * Four Tier-0 built-ins live (default-layout, welcome, ipc-status,
    theme-picker) plus 17 id-reserving stubs so later tiers fill in
    without rename churn.

.gitignore extended to cover app/ sub-package artefacts and the
Playwright harness scratch dirs.

Co-Authored-By: Claude <noreply@anthropic.com>

.gitignore

@@ -16,6 +16,25 @@
 /bin/clide.exe
 # pubspec.lock is committed (supply-chain policy).
 
+# Flutter app sub-package under app/. Same patterns, different prefix.
+app/.dart_tool/
+app/.flutter-plugins
+app/.flutter-plugins-dependencies
+app/build/
+app/*.iml
+app/ios/Pods/
+app/macos/Pods/
+app/windows/flutter/ephemeral/
+app/linux/flutter/ephemeral/
+app/macos/Flutter/ephemeral/
+
+# UI harness (Playwright) — npm + output artefacts.
+tools/ui/node_modules/
+tools/ui/out/
+tools/ui/test-results/
+tools/ui/playwright-report/
+tools/ui/.serve.pid
+
 # -- ptyc (C supporter tool) --------------------------------------------
 /ptyc/bin/
 /ptyc/*.o