docs: add icon composition brainstorm, plan, and terminology-rename stub

- brainstorm: 2026-04-17-dtpr-icon-composition-brainstorm
- plan: 2026-04-17-002-feat-dtpr-icon-composition-plan (the blueprint
  this branch implements)
- stub: 2026-04-17-001-feat-content-release-terminology-rename
  (deferred follow-up)

Author: pichot