[Feature Request] createDatePicker: headless date primitive (composable-first)

[johnleider]

Build the createDatePicker composable — headless date state built composable-first (no @internationalized/date, leaning on the useDate adapter): calendar-grid geometry, range mode, presets, min/max, disabled dates.

Part of the v1.5.0 epic (#511). The DatePicker and DateRangePicker component shells build on this.

Category forms and the name createDatePicker are provisional — adjust at implementation.

Checklist — new-feature checklist, composables

• packages/0/src/composables/createDatePicker/ — index.ts (+ index.test.ts)
• packages/0/src/composables/index.ts — add export * from './createDatePicker' (alphabetical)
• packages/0/src/maturity.json — add createDatePicker entry: level: preview, since: null, category: forms, description
• apps/docs/src/pages/composables/forms/date-picker.md — docs page with frontmatter
• apps/docs/src/examples/composables/date-picker/basic.vue — basic example
• apps/docs/src/pages/composables/index.md — table entry (forms section)
• packages/0/README.md — What's Included table
• root README.md — keep in sync
• packages/0/src/surface.test.ts — add exports to COMPOSABLES freeze array (sorted)
• <DocsApi /> renders + feature appears in api-whitelist.ts after build
• pnpm build:0 && pnpm typecheck && pnpm lint:fix && pnpm build:docs green