0019. How a frozen document is superseded

Context

ADR-0001 and ADR-0018 freeze ratified documents, and both name superseded as a later state. A reader who opens an old document needs to see that it has been replaced, and the class index needs to say so too. Editing the frozen file in any other way must stay impossible.

Decision

Supersession is recorded in three places, in the same pull request:

1. The new document carries supersedes: <old number or path> in its frontmatter. It is a new file, which the immutability check always allows.
2. The old document receives a supersession stamp: its status: changes to Superseded (ADRs) or superseded (proposals, specs), optionally with a superseded-by: frontmatter field. Nothing else in the file may change. The docs-immutability check allows exactly this edit and nothing more: the body and every other frontmatter line must stay byte-for-byte identical, and a superseded document is frozen again.
3. The class index (adr/README.md, proposals/README.md, specs/README.md) marks the old entry superseded by NNNN. Each class MUST keep such an index; it is the place to look up a document's current status.

Consequences

• An old document shows on its own page that it was replaced, and the index agrees.
• The record itself never changes, so references to it stay trustworthy.
• Every ratification or supersession includes an index edit, which is always allowed.