Tighten type inference and remove v1 escape hatches

[yamcodes]

In #704 , we intentionally loosened types to unblock v1:

• any in:
  • createEnv(def: unknown)
  • coerce(schema: any)
  • plugin calls (options as any)
• FilterByPrefix bailout to any extends T ? any : …
• InferType doing a lot of conditional gymnastics

Scope:

1. Audit any usage added for Standard Schema support
2. Replace any with constrained generics where feasible
3. Revisit InferType complexity and collapse cases
4. Improve plugin typing without deep-instantiation explosions

[yamcodes]

Closing since this was a follow up from #705 which is closed in favor of #722