Client/server Data Strategy

[bart-krakowski]

Parent

#25

What to build

Add a pluggable `strategy` option to the kernel (inspired by AG Grid's row model). In `'client'` mode all pipeline stages run in-process. In `'server'` mode the delegatable stages — recurrence-expand, conflict-detect, slot-generation — run server-side via the same isomorphic Validation Core; the client receives materialized results and only runs clip-to-viewport and layout.

The consumer's calendar API is identical in both modes. The observable output (`Day[]`, event layout, conflict arrays) must match regardless of which strategy is active.

This is computation push-down, not just data-sourcing. The boundary is serializable for free because ADR-0002 restricts all public types to native `Date`, `YYYY-MM-DD` strings, and plain objects — no Temporal instances, no class instances.

The kernel emits a serializable operation request per delegatable stage (e.g. `{ op: 'expandRange', range, events, config }`). This request is also the TanStack Query key, so caching and invalidation require no extra plumbing. The consumer provides a `transport` function that forwards the request to their endpoint, which calls the matching server-side handler from `@tanstack/time-server`.

In `'client'` mode advisory and authoritative validation collapse — the client is the only validator. In `'server'` mode the client runs advisory checks; the server is authoritative.

Acceptance criteria

• `createCalendar({ strategy: 'client' })` behaves identically to the current in-process behaviour
• `createCalendar({ strategy: 'server', transport })` forwards delegatable stages to the consumer's `transport` function
• `transport` receives a serializable operation request and returns a plain-object result; it owns the fetch/RPC/auth
• The operation request object is stable and suitable as a TanStack Query `queryKey`
• `@tanstack/time-server` exports a handler per delegatable operation that the consumer's endpoint calls
• Given the same event data, `strategy: 'client'` and `strategy: 'server'` (with a mock transport) produce identical `Day[]` output
• Clip-to-viewport and layout always run client-side regardless of strategy

Blocked by

#28
#37