Development record

`04-surfaces/34-build.md:295` says the state is schema version 20, and line 383 says an earlier version is written back at version 20 by its next mutation. `internal/core/implement/loop/state.go:281` declares `SchemaVersion = 21`, and state.go:285 keeps 20 as read and never written. Version 21 added a round's `resolves_shown` and a landing's `gate_unread_since` (state.go:271-276), which no brief chapter names.