docs(plan): requirements slice 1 — requirement checks and requirement-test generators in every port (#406)

Implementation plan only, no code. Covers the two shared corpora (requirement-check
and requirement-test identity) run by the TypeScript reference, the requirement gate
in each port's verify, a requirement-tests generator per port (pytest, JUnit, xUnit)
with project-owned witness functions, the three seams (per-member grain, a renderer
hook that receives the requirement digest, a require-implementers switch), and the
ADR and docs change that reverses the TypeScript-only decision.

Author: dmealing