vector types · phase 8: language docs

Phase 8 of docs/discussions/design-vector-types.md. docs/lang/types.md §8 already states decisions 1, 2, 6 and the pointer rule. Add:

• the three operations (vector load, splat, reinterpret);
• the alignment rule: one alignment on the type, under-alignment carried by the pointer;
• the unsafe surface, including unsafe fn and the Rust-2024 body rule;
• vectors in ordinary structs, and the extern "C" refusals;
• the placement sentence: unsafe does not unlock a memory space.

Blocked on Vx#479 and Vx#480.

Start with docs/discussions/design-vector-types.md and the existing §8 in docs/lang/types.md, then check Vx#479 and Vx#480 because this phase is blocked on them. Update §8 with the three operations, alignment and unsafe rules, struct and extern "C" behavior, and placement sentence; done means all listed decisions are documented consistently.