feat: zod v4 uses spread / extend

[mnahkies]

previously we were using a combination of the deprecated merge and intersection for creating intersected types.

now we'll

• Continue to use z.intersection if any of the candidates isn't an object
• Use s_Thing.extend({ /* properties*/ }) if combining a named schema with anonymous additions
• Use z.object({ ...s_Thing.shape, ...s_OtherThing.shape }) otherwise

this removes the use of the deprecated merge, and should be more efficient according to the zod documentation, and generally more idiomatic usage of zod.

also takes some steps to "squash" unnecessary intersection types - eg: {a: string} & {b: string} should squash to simply {a: string, b: string}