Explain change

Produce one self-contained HTML packet so teammates understand a resolved git range without reading docs/specs/* or CONTEXT.md as human prose. Specs and glossary stay the agent map; this file is a derived human projection.

Aid for shared understanding — never a merge, PR, or cut-cut-release gate.

The Iron Law

ONE SUCCESS PATH: docs/explainers/<slug>.html written (overwrite) AND
  docs/explainers/INDEX.md upserted AND the written file verified
OR
ONE HONEST HARD-STOP: message only — no partial HTML, no fake INDEX success