RFC #735: schema $id / URI strategy for spec JSON schemas

## Context

Open question from RFC #735 (Spec Versioning and Evolution), tracked in `docs/rfc-coordination.md`.

Current JSON schemas use `v0` path prefixes de facto (e.g. `https://opensource.adobe.com/spectrum-design-data/schemas/v0/token.json`). A formal policy is needed: whether `v0` is permanent or migrates to `v1` on 1.0.0 release, how the URI maps to the published spec version, and what happens during the two-minor migration window for breaking schema changes.

## References
- RFC #735: https://github.com/adobe/spectrum-design-data/discussions/735
- `docs/rfc-coordination.md` RFC #735 row
- `spec/evolution.md`

Start by reading RFC #735, the RFC #735 row in docs/rfc-coordination.md, and spec/evolution.md to understand the versioning constraints and migration window. Done means documenting and aligning a settled $id/URI policy with the referenced RFC and schema-versioning guidance.

Written by the indexing model from the issue text.