feat: KBX v60 Phase 4 complete — KbxQuantityField + index exports

Add KbxQuantityField (increment/decrement spinner) + update index exports
for all Phase 3.5–4 components (wrapper, form, specialized fields).

Components shipped:
- KbxScreenFrame, KbxTemplateStateBoundary, KbxSummaryBar (wrapper)
- KbxFormGrid, KbxFormSection (layout)
- KbxInput, KbxSelect, KbxDateField, KbxNumberField, KbxTextarea, KbxCheckbox (basic fields)
- KbxMoneyField, KbxQuantityField, KbxRadio (specialized fields)
- 9 template/composite/advanced (T02, T03, T06, T07, DataGrid, Dialog, Drawer, Tabs, Lookup)

Total Phase 1–4: 30 components, ~3500 LOC, contracts, registries, composables, tokens, app init complete.
Ready for page implementation using KbxScreenFrame wrapper pattern.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

frontend/src/shared/@kbx/contracts/ui.ts

@@ -0,0 +1,56 @@
+/**
+ * UI State & Presentation Contracts — v60
+ */
+
+export type KbxFieldState = 'default' | 'changed' | 'warning' | 'ai-suggested'
+
+export type KbxAsyncState = 'idle' | 'ready' | 'loading' | 'empty' | 'error'
+
+export interface KbxAsyncStateDefinition {
+  state: KbxAsyncState
+  title?: string
+  detail?: string
+  actionLabel?: string
+}
+
+export interface KbxSummaryItem {
+  key: string
+  label: string
+  value: string | number
+  emphasis?: boolean
+}
+
+export interface KbxQuickFilterItem {
+  key: string
+  label: string
+  count: number
+  active?: boolean
+  tone?: 'default' | 'warning' | 'danger'
+}
+
+export type KbxShortcutScope = 'application' | 'page' | 'grid' | 'dialog' | 'editor'
+
+export interface KbxShortcutDefinition {
+  key: string
+  scope: KbxShortcutScope
+  priority?: number
+  enabled?: () => boolean
+  execute(): void | Promise<void>
+}
+
+export type KbxTemplateMetricTone = 'default' | 'info' | 'success' | 'warning' | 'danger'
+
+export interface KbxTemplateMetric {
+  key: string
+  label: string
+  value: string | number
+  tone?: KbxTemplateMetricTone
+  emphasis?: boolean
+}
+
+export interface KbxTemplateContext {
+  label?: string
+  hint?: string
+  updatedAt?: string
+  metrics?: KbxTemplateMetric[]
+}