Refactor validation logic to lazily initialize TypeBox compiler, improving performance in CSP-restricted environments.

[samuelstroschein]

Note

Introduces a lazy, CSP-safe validation strategy for Zettel AST.

• Refactors validate.ts to lazily compile a TypeBox validator on first use; falls back to Value.Check via TypeCheck when code generation/eval is blocked (e.g., Cloudflare Workers)
• validate() now uses a cached validator and derives errors from validator.Errors
• Adds validate.test.ts to assert lazy compilation and fallback behavior
• Adds a changeset for a minor release of @opral/zettel-ast

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