loom-code-migration

# Code Migration ## Overview Move a codebase between versions, frameworks, technologies, or schemas while keeping it shippable and reversible at every step. Distinct from `/loom-refactoring` (behavior-preserving, internal) — a migration deliberately *changes* what runs, so it needs a parity harness and a rollback path. ## The mandate: never big-bang A big-bang cutover (rewrite in a branch, flip everything at once) fails because the blast radius equals the whole system, the parity gap is unmeasured until launch, and rollback means reverting weeks of work under fire. **Every migration step must be independently deployable and independently reversible.** If you can't ship the current step to prod and roll just it back, the step is too big. ## Canonical incremental playbook Every strategy below is an instance of this loop. Internalize the loop, not the individual recipes. ```text 1. PARITY HARNESS Pin current behavior: characterization/snapshot tests, or a live shadow-compare of old vs new. You cannot migrate safely what you cannot measure. 2. SEAM Insert an indirection point you can route through: branch-by-abstraction (interface), adapter, or strangler proxy. 3. BUILD NEW Implement the new path behind the seam. Old path still serves. 4. ROUTE Shift traffic incrementally behind a flag: shadow → canary(1%) → gradual(5→25→50→100%). Fallback to old on error. 5. VER...