[Docs] wasm is threaded: update dispatchers and platform tables #5

Decisions of 2026-10-04: free-threaded CPython 3.15t is the only build (python-multiplatform #158), and wasm is also 3.15t with pthreads under cross-origin isolation by default (python-multiplatform #175). SPEC S3.2, S7.7 and research §6/§8 still say wasm has no threads and Default/IO raise NotImplementedError. Update them: Default and IO run on worker threads in an isolated page; record what happens when the page is not isolated (the runtime refuses to start, per #175). Spec only.