Collapse column-mapping stack; break FieldStorageContract extension API pre-1.0

Status: accepted

The Phase-5 storage stack had five participants for one concept (“where does a typed value live in the row?”): ColumnMapping, FieldStorageContract, CurrencyStorageContract, Field instance methods (read_value / write_value / apply_default_to), and the storage_contract_class macro. FieldStorageContract was a pass-through wrapper for 19 of 20 field types — one real adapter (Currency) on a seam designed for many. Currency carried double-declarations across the Field class and its dedicated contract subclass, where the same column list lived in three places.

We collapsed the stack into a single Field::TypedStorage concern that lives on Field::Base. The contract classes, the storage_contract_class macro, and field.storage_contract are removed. Multi-cell field types override three instance methods on their Field subclass (read_value, write_value, apply_default); snapshot/change-detection methods become concrete (derived from value_columns). Currency becomes a normal Field subclass with overrides — the same shape external custom multi-cell types now use.

Considered alternatives

• α (strict BC): keep FieldStorageContract and storage_contract_class as the public extension surface; rewire internal callers to use Field methods directly; delete only CurrencyStorageContract. Achieves the internal cleanup without breaking external authors. Rejected because the wrapper class would survive as legacy plumbing with no internal justification, leaving the documented extension API and the actual internal pattern divergent forever.
• β (deprecate with warnings): same as α, plus deprecation warnings on storage_contract_class / field.storage_contract. Rejected because CONTEXT.md’s “BC is binding” rule provides no on-ramp to removal — the deprecation would point at nothing and become permanent noise.
• γ (chosen): break BC. Remove FieldStorageContract entirely. Re-ship as 0.3.0.

BC reconciliation

CONTEXT.md (.vbw-planning/CONTEXT.md) lists “Backwards compatibility is binding” as a key decision. That rule was scoped to the Phase 1–7 enhancement arc — every phase preserves current API surface. This refactor is not a phase; it’s an out-of-band architectural cleanup happening pre-1.0 where SemVer allows breaking changes at the minor version. Future architectural refactors with the same character should follow the same pattern: an explicit ADR + a minor-version bump, not a silent break.

Consequences

• External authors who subclass FieldStorageContract must migrate to override-on-Field (subclass Field::Base, override value_columns / operator_column / read_value / write_value / apply_default). The Currency source serves as the canonical example.
• The README’s §”Multi-cell field types” is rewritten around the new pattern.
• Three spec files (field_storage_contract_spec, column_mapping_spec, column_mapping_value_columns_spec) consolidate into one field/typed_storage_spec.
• Override surface shrinks from 7 methods to 3. Snapshot shape becomes a versioning-coupled invariant locked in by value_columns rather than an extension point external authors could silently break.