ADR-009: Requirements are superseded, never mutated

Controlled environments (IEC 62304, ISO 13485) require baselined requirements to be immutable: a change creates a successor, it never edits the old item in place. Antora Tracer had no way to express "this requirement replaced that one", and nothing surfaced the designs and tests that still pointed at the old item. Two plausible mechanisms existed — a status attribute or an explicit relationship — and they had different drift and queryability properties.

Chosen option: a supersedes relation (successor → predecessor), with the predecessor’s superseded state derived from the graph — an item is effectively superseded when at least one incoming supersedes relation targets it. Splits and merges are first-class; self-supersession, duplicate links, and cycles are validation errors; a functional link to a superseded item is an advisory stale_link warning, and current-state matrices omit superseded items.