Update planning package after Phase 0B

planning/DECISIONS/005-branch-preserving-history.md

@@ -1,29 +1,44 @@
 # ADR 005 — Branch-Preserving Story History
 
-**Status:** Accepted in principle; implementation pending Phase 0
+**Status:** Accepted; implementation direction validated in Phase 0B
 
 ## Decision
 
-Returning to an earlier story point should preserve abandoned future history as another branch rather than destructively erasing it.
+Returning to an earlier story point preserves abandoned future history rather than destructively erasing it.
+
+The selected implementation model uses a **movable active head over retained lineage**:
+
+- Undo moves the head backward,
+- Redo moves it forward along the retained continuation,
+- accepted turns are not deleted by ordinary Undo,
+- a new write after moving backward creates a new continuation on first divergent write,
+- the displaced future remains retained/disposable,
+- normal UI presents Undo/Redo/Retry/Save Point rather than branch-management concepts.
 
 ## Context
 
 The user must be able to recover from unwanted story developments and explore alternatives while retaining prior work.
 
+Phase 0B found that AI-DnD's shipped Undo was destructive even though its tree/take infrastructure was otherwise strong. A disposable spike demonstrated non-destructive head-cursor Undo/Redo using existing lineage chokepoints and branch creation while preserving branch-scoped memory isolation.
+
 ## Alternatives Considered
 
 - destructive undo,
 - overwrite-in-place editing,
 - complete copy of campaigns for every retry,
-- branch-preserving turn graph.
+- branch-preserving turn graph with movable active head.
 
 ## Reason
 
-A branch-preserving history provides recovery, experimentation, and auditability without unnecessary campaign duplication.
+The selected model provides recovery, experimentation, auditability, and Redo without unnecessary campaign duplication or a complicated user-facing branch workflow.
 
 ## Consequences
 
-- turn identity/parentage must be first-class,
-- state restore must be branch-aware,
-- retry/edit semantics must be explicitly defined,
-- the selected candidate repository must either support this or be adaptable to it.
+- turn identity/parentage remains first-class,
+- active head and retained tip are distinct concepts,
+- state restore is lineage-aware,
+- retry/edit/add-take must honor fork-if-behind-head behavior,
+- summaries/memories must respect active lineage/head,
+- named checkpoints point to durable story positions,
+- abandoned history is marked disposable but is not automatically cleaned up in v1,
+- export/import must preserve the active head coordinate as well as the retained history graph.