Raise E_SCHEMA_VERSION_MISMATCH when composing a fragment #665

Part of #664.

Composition admits fragments by schemaVersion range (chapter 40, the version table), but nothing raises E_SCHEMA_VERSION_MISMATCH. The code is declared in src/model/diagnostic.ts, and scripts/lint-codes.ts lists it as pending under #204, which closed without it.

So today a fragment written for a newer minor composes. Fields the composing toolkit doesn't know are dropped, and the run still exits zero. That is the silent loss the chapter's "Minor > toolkit stays a hard stop" paragraph warns about.

What

• In composition, compare each fragment's manifest.spec.schemaVersion with the toolkit's:
  • a different major, in either direction, is E_SCHEMA_VERSION_MISMATCH;
  • the same major with a higher minor is E_SCHEMA_VERSION_MISMATCH;
  • a patch difference is admitted.
• The message names the rule that fired, the fragment and its digest.
• As a participant's own error, it isolates that participant at its last composed fragment even if the digest did not change (chapter 40, "A participant's own errors isolate it").
• Remove the code from the Slice 7: every refusal through check/, the pending codes included #204 pending list in lint-codes.ts.

Why after 1.0.0

At 1.0.0 every fragment is written at 1.0.0, so nothing can trip the gate yet. It matters from the first minor release after that. The version-rollout steps in #381's runbook describe the gate as specified.