v4.0.0

Any two versions of an instruction set can now be compared with a structured, step-level diff: added and removed steps, per-field before and after on changed ones, and the section entries that moved. It reads the stored snapshots, so it works for history as well as the head.

Branches let an expert experiment without touching a live set. POST /branch copies a version (the published one by default) into a new draft with its own edits, test runs and review; POST /merge three-way merges it back into the parent head, taking one-sided changes and returning a 409 with the exact conflicting steps and fields when both sides changed the same thing. Migration 0004 adds the branch columns. 44 tests.