Prefactor: extract Validation Core from CalendarCore

[bart-krakowski]

Parent

#25

What to build

Extract the availability, dependency-propagation, and recurrence validation logic currently fused into `CalendarCore` into pure, isomorphic functions with the signature `validate(write, events, config) → conflicts`. These functions must have zero dependencies on the store, viewport, DOM, or any database.

This is the mandatory first step before any modularization can happen. It is a pure refactor — no new behavior, no public API changes. The existing `CalendarCore` continues to call these functions internally; callers see no difference.

The extraction creates the seam that makes the following possible:

• the module-stage pipeline (both client and server can run the same stages)
• the `loadEvents` adapter in the server package
• the client/server Data Strategy

Acceptance criteria

• Availability conflict detection, dependency-constraint checking (FS/SS/FF/SF), and recurrence validation each exist as standalone pure functions importable independently of `CalendarCore`
• Each function accepts plain event objects and config; returns a conflicts/errors array; has no side effects
• `CalendarCore` delegates to these functions internally — all existing tests continue to pass unchanged
• No `Temporal` objects, store references, or DOM references appear in the extracted functions' signatures
• Unit tests cover the extracted functions directly (not through `CalendarCore`)

Blocked by

None — can start immediately.