Server package: validateOnServer + loadEvents adapter

[bart-krakowski]

Parent

#25

What to build

Build `@tanstack/time-server` — a runtime-agnostic package that exposes the shared isomorphic Validation Core with a `loadEvents` adapter seam for authoritative server-side validation. The library never opens a database connection; the consumer implements `loadEvents` against their own DB inside their own write transaction.

The package's key export:

```ts
validateOnServer(write, { loadEvents }, ctx) → Promise<conflicts[]>
```

The library computes `getRequiredRange(write)`, calls `loadEvents(range, ctx)` to get the relevant events from the consumer's DB, then runs the same pure Validation Core as the client's advisory checks. Same logic, authoritative data.

Documentation must prominently state: read-validate-write is only race-free inside one transaction with a consistent/locked view. The library provides rules; the consumer provides isolation. A DB exclusion constraint is the recommended backstop for raw time-overlap.

Acceptance criteria

• `@tanstack/time-server` is a new package in the monorepo with no DOM or browser dependencies
• `validateOnServer(write, { loadEvents }, ctx)` calls `loadEvents` with the required range, then runs the Validation Core
• `loadEvents(range, ctx) → Promise<Event[]>` is the only DB seam; the package opens no connections
• The package works in Node, Bun, Deno, and edge runtimes (no Node-specific APIs)
• Unit tests use a mock `loadEvents` — no real DB required
• The package exports the same Validation Core pure functions directly for consumers who want to call them without the adapter orchestration
• Documentation clearly states the transactional-isolation requirement and the DB exclusion constraint recommendation

Blocked by

#26