dissolve app/ into repo root (D-056)

Single Flutter package at the repo root. All code, tests, assets,
and platform directories moved from app/ to root. Package renamed
from clide_app to clide — all imports rewritten. Merged pubspec
combines core (ffi) and app (flutter, yaml, xterm) dependencies.
Makefile simplified: no APP_PRESENT conditionals, no cd, no daemon
lifecycle. 317 tests pass.

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

lib/extension/src/contribution.dart

@@ -0,0 +1,157 @@
+import 'package:clide/clide.dart';
+import 'package:clide/kernel/src/panels/slot_id.dart';
+import 'package:flutter/widgets.dart';
+
+/// One atom contributed by a [ClideExtension]. Extensions declare N of
+/// these in a manifest; the kernel and slot hosts render them.
+///
+/// Adding a new contribution type: add a case to this sealed hierarchy,
+/// extend the host dispatch in the default-layout extension, and bump
+/// the extension manifest schema version.
+sealed class ContributionPoint {
+  const ContributionPoint({required this.id});
+
+  /// Stable id for this contribution, unique within its extension.
+  final String id;
+
+  /// The slot this contribution targets, or `null` for non-slot
+  /// contributions (commands, events, grammars).
+  SlotId? get slot => null;
+}
+
+/// A tab in a slot that hosts tabs (sidebar / workspace / context).
+class TabContribution extends ContributionPoint {
+  const TabContribution({
+    required super.id,
+    required this.slot,
+    required this.title,
+    required this.build,
+    this.icon,
+    this.priority = 0,
+    this.fileGlobs = const [],
+    this.listenable,
+    this.titleKey,
+    this.i18nNamespace,
+  });
+
+  @override
+  final SlotId slot;
+  final String title;
+  final WidgetBuilder build;
+  final Object? icon;
+  final int priority;
+  final List<String> fileGlobs;
+  final Listenable? listenable;
+
+  /// When set, the slot host resolves the display title via
+  /// `i18n.string(titleKey, namespace: i18nNamespace, placeholder: title)`.
+  /// [title] stays as the English fallback (also used in tests/logs).
+  final String? titleKey;
+
+  /// The i18n namespace to look up [titleKey] in. Extensions usually
+  /// pass their own `id`. Required when [titleKey] is set.
+  final String? i18nNamespace;
+}
+
+/// A status-bar item. Order is determined by [priority] within each
+/// alignment group; negative priorities float left, positive right.
+class StatusItemContribution extends ContributionPoint {
+  const StatusItemContribution({
+    required super.id,
+    required this.build,
+    this.priority = 0,
+    this.listenable,
+  });
+
+  @override
+  SlotId get slot => Slots.statusbar;
+  final WidgetBuilder build;
+  final int priority;
+  final Listenable? listenable;
+}
+
+/// A button in the main toolbar.
+class ToolbarButtonContribution extends ContributionPoint {
+  const ToolbarButtonContribution({
+    required super.id,
+    required this.label,
+    required this.onPressed,
+    this.icon,
+    this.tooltip,
+    this.priority = 0,
+  });
+
+  @override
+  SlotId get slot => Slots.toolbar;
+  final String label;
+  final Object? icon;
+  final String? tooltip;
+  final int priority;
+  final VoidCallback onPressed;
+}
+
+/// A command extensions register with [CommandRegistry]. Surfaced by the
+/// command palette, the keybinding resolver, and `clide` CLI subcommands.
+class CommandContribution extends ContributionPoint {
+  const CommandContribution({
+    required super.id,
+    required this.command,
+    required this.run,
+    this.title,
+    this.defaultBinding,
+  });
+
+  final String command; // e.g. "git.commit"
+  final String? title; // "Git: Commit staged"
+  final String? defaultBinding; // e.g. "ctrl+shift+g"
+  final Future<IpcResponse> Function(List<String> args) run;
+}
+
+/// Registers an item in the OS tray / menu-bar.
+class TrayItemContribution extends ContributionPoint {
+  const TrayItemContribution({
+    required super.id,
+    required this.label,
+    required this.onSelected,
+    this.priority = 0,
+  });
+
+  @override
+  SlotId get slot => Slots.tray;
+  final String label;
+  final int priority;
+  final VoidCallback onSelected;
+}
+
+/// A named layout arrangement. One "classic" preset ships with
+/// `builtin.default-layout`; other presets can be contributed.
+class LayoutPresetContribution extends ContributionPoint {
+  const LayoutPresetContribution({
+    required super.id,
+    required this.displayName,
+    required this.slots,
+  });
+
+  final String displayName;
+  final List<LayoutSlot> slots;
+}
+
+/// One slot in a [LayoutPresetContribution]. Describes where the slot
+/// appears and its initial size/visibility.
+class LayoutSlot {
+  const LayoutSlot({
+    required this.slot,
+    required this.position,
+    this.defaultSize,
+    this.minSize,
+    this.maxSize,
+    this.visible = true,
+  });
+
+  final SlotId slot;
+  final SlotPosition position;
+  final double? defaultSize;
+  final double? minSize;
+  final double? maxSize;
+  final bool visible;
+}