Export Zod schemas to a shared validation package

[Jayy4rl]

Context: Frontend applications want to validate API responses client-side using the same schemas the backend uses for request validation.

Requirements:

• Create sdk/packages/validation/src/index.ts re-exporting: VaultSchema, EpochSchema, UserSchema, PositionSchema.
• Schemas must match the API response shapes exactly. Publish as @stellaryield/validation within the monorepo.

Acceptance criteria:

• VaultSchema.parse(apiResponse) succeeds for a valid vault response.
• VaultSchema.parse({ invalid: true }) throws a ZodError.