Append-only

Key features

• Native mode — on a checkout carrying dev/session-branch-surface.patch (Session.selectMessageSurface()), jump/fork switch the model-visible history directly: the next turn genuinely starts from the new branch.
• Append-only — every node is immutable
• Stock mode — on an unmodified official Harness, the plugin emulates the switch through official append APIs (an empty replace surface event) plus a plugin-owned sidecar under $DSH_HOME/storages/session-tree/ for durable branch state.
• Projection mode — if even the emulation is unavailable, navigation moves the tree projection and panel only, and the next turn keeps the canonical history.
• branching and jumping only move the cursor.