docs: close out S0 (Foundation) — update living docs

[sepehr-safari]

Milestone-boundary documentation audit for S0 — Foundation.

Now that the scaffold (#1), CI (#2), founding docs (#3), and ADRs (#4) have landed, update the living docs to reflect reality:

• CURRENT_STATE.md — move founding docs + ADRs to What's done; clear What's in progress; set What's next to S1; mark the docs area done for S0.
• ROADMAP.md — mark S0 as ✅ Complete with its exit criteria met (CI green on macOS + Linux, native doctor --strict clean).

Acceptance criteria

• CURRENT_STATE.md reflects S0 complete and S1 next
• ROADMAP.md shows S0 ✅ Complete
• No stale 'in progress' references to merged work

Milestone: S0 — Foundation