fix(react-vtable): lazy load custom layout reconciler

[fangsmile]

What

Fix React 19 + Vite rendering failure when using simple @visactor/react-vtable ListTable.

Closes #5203

Root Cause

@visactor/react-vtable exports CustomLayout from the package entry. CustomLayout statically imported ./reconciler, which initialized react-reconciler at module load time.

In a React 19 + Vite app, even a simple ListTable import could therefore load the React 18 matched react-reconciler@0.29.0 during dependency prebundle and fail with:

Cannot read properties of undefined (reading 'ReactCurrentOwner')\n```\n\n## Changes\n\n- Lazy-load `CustomLayout` reconciler only when `CustomLayout` is actually rendered.\n- Keep normal `ListTable` usage independent from the custom-layout reconciler path.\n- Add a minimal React 19 + Vite regression demo for issue #5203.\n\n## Verification\n\n- Reproduced before the fix with `vite.config.issue5203.ts` + `#issue5203ViteReact19`:\n  - blank page\n  - no table/canvas\n  - `ReactCurrentOwner` console exception from `custom/reconciler.ts`\n- Verified after the fix with the same demo:\n  - `ready=true`\n  - `hasTable=true`\n  - `canvasCount=1`\n  - no console exception\n- Verified existing React 19 `customLayout` demo still works:\n  - `demoError=null`\n  - `hasTable=true`\n  - `canvasCount=1`\n- `git diff --check` passed.\n\nNote: `npm run compile` in `packages/react-vtable` is currently blocked in this worktree because `@visactor/vtable` package type outputs are not generated/resolved (`Cannot find module @visactor/vtable...`). This is an existing package build-order/environment issue, not introduced by this change.