Assemble the build-ready spec

## Question

Assemble everything this map settled into one build-ready spec, and hand it off.

The destination. Gather the resolutions, the two prototypes, the OCR research, the schema and route map into a single document that someone — or some agent — can build the MVP from without reopening any of it.

- **Publish** at `.scratch/tabby/spec.md`.
- **Carry** the domain model by reference to `CONTEXT.md` rather than restating it, and link ADR-0001 for the derived-balance decision.
- **Cover** the claim flow end to end, the balance and settlement flow, the schema, the route and permission map, and the UX states the prototypes established.
- **State the scope boundary explicitly** — the map's Out of scope section, plus anything ruled out along the way — so the build does not quietly re-expand it.
- **Hand off** to `/to-tickets`.

The map is done when this closes.

Start with CONTEXT.md and ADR-0001, then gather the map's resolutions, two prototypes, OCR research, schema, route map, and Out of scope section. Assemble the claim, balance, settlement, UX, permissions, and scope decisions in .scratch/tabby/spec.md, and hand it off to /to-tickets when the map is fully represented without reopening decisions.

Written by the indexing model from the issue text.