PR-B1 widens this from never (PR-A's forward-compat placeholder)
to the six variants the EPIC-2 passes actually consume. The
discriminator is kind, with on-the-wire literal strings that
must match the Rust serde rename byte-for-byte:
subscribesubscribe-callback (kebab; sixth variant for commit-origin
lineage per the brutal-critical review's recommendation #1)unsubscribedisposereadtx-set (kebab; the engine-facing TS uses tx.set and the
wire byte is the kebab-case form)Adding a seventh variant requires changing this declaration and is
caught at every assertNever-guarded reading site in the engine
and the checker.
Lifecycle event variant — the closed six-arm discriminated union EPIC-2's lint passes pattern-match against.