vector types · phase 0: confirm the design against the tree, and lock the four amendments

Phase 0 of docs/discussions/design-vector-types.md (tracked in the meta issue, linked below): investigate and report, no code. Most of the seven questions are answered already (Vx#472, the review comment); what remains is listed in the implementation plan at docs/discussions/implementation_plans/vector_types_phase0.md, together with the four amendments (A: spell the cast with as; B: a vector inside a struct takes natural alignment, element alignment is for pointers; C: alignment of a tensor-derived pointer comes from the allocator's stated guarantee; D: user unsafe fn is a prerequisite or the callee vouches with a block).

Deliverable: a "Phase 0 report" section appended to the design document with each numbered answer and a file:line, the four amendments written into its Section 2, and a note here. The checkpoint in the design's Section 9 is that report.

Read docs/discussions/design-vector-types.md and docs/discussions/implementation_plans/vector_types_phase0.md first, then confirm the seven numbered answers against the repository tree. Done means appending a Phase 0 report with file:line references, recording amendments A–D in Section 2, and noting completion here; no code is requested.