Composition admits a fragment one schema major behind the toolkit #438

What to build

Composition admits a fragment one schema major behind the composing toolkit, within a stated window, so a toolkit major does not strand every fragment published before it until each application repository publishes again.

Today spec/v1/40-composition.md refuses any other major. With the Estate repository as the one toolkit pin, a minor upgrade needs no application repository change; a major one still does, and this closes that gap.

Acceptance criteria

• spec/v1/40-composition.md states the window and what composition does with a fragment inside and outside it.
• Both implementations, with oracle files.

No major is planned, so this waits until one is.