Replace the value/valueType/originalValue/originalValueType fields on makeColumnField() contexts with an explicit field object API for reading, writing, resetting, comparing and validating field values.
Normalize columnType to {type, config}, rename the SQLite default metadata so it is clearly SQLite-specific, and document that plugins submit only string, number, boolean or null values. Plugins that need structured data should serialize it themselves instead of relying on Datasette to special-case JSON.
Move the built-in json column type behavior onto the same plugin API used by external plugins: validate the textarea with field.setValidity() as the value changes, but submit plain text. Harden row edit value comparison so fixing invalid JSON in an existing row is not blocked by the original invalid value.
Update the JavaScript plugin documentation and Node-based tests for the revised field contract.
