Pure function that computes a v2 → v1 category migration plan from a
snapshot of the profile data. The 4-pass algorithm (keyword → supplier
propagation → default fallback → needs review) produces a MigrationPlan
with confidence badges (high/medium/low/none) per row, exposes split
targets for categories that ventilate across multiple v1 leaves (e.g.
Transport en commun → Autobus + Train), and preserves user-custom
categories in a dedicated bucket for later placement under
"Catégories personnalisées (migration)".
- Mapping tables encoded from
.spikes/archived/seed-standard/code/mapping-old-to-new.md
- No DB I/O: the caller hands us categories, keywords, transactions and
optional suppliers; the service stays testable and side-effect-free.
- 20 unit tests cover every pass, custom preservation, split exposure,
stats aggregation and pass priority.
Prepares the ground for #121 (migration writer UI).
