spec(schema/mysql): define column option and mode contracts

[scottescue]

Outcome

Ratify the shared Go, snapshot, code-generation, and DDL contract for MySQL temporal, numeric, decimal, and integer column options before feature builders are implemented.

Context

The schema specification identifies FSP, precision/scale, unsigned, and upstream mode gaps but does not yet define all Go option shapes or mode-to-value mappings. Temporal, decimal, and bigint features must consume one consistent policy.

Decisions required

• Typed option shapes and validation bounds for FSP, precision, scale, and unsigned.
• timestamp, datetime, and date mode mappings to Go values/codecs.
• decimal mode mappings and precision/scale interaction.
• bigint mode and signed/unsigned Go representations, including overflow boundaries.
• Snapshot fields, zero-value/omitted-option semantics, compatibility, and deterministic DDL ordering.
• Behavior for existing PostgreSQL builder re-exports that cannot represent MySQL-specific options.

Out of scope

• Implementing builders or code generation before ratification.
• Query arithmetic and driver behavior unrelated to the selected column value representation.
• Serial/autoincrement redesign.

Ratification criteria

• schema.md, codegen.md, and types.md agree on every option and mode mapping.
• Go option APIs, accepted bounds/combinations, and omitted-value behavior are explicit.
• Snapshot representation and backward-compatibility rules have deterministic examples.
• Generated Go values/codecs and MySQL DDL are specified for every mode and signedness choice.
• Temporal, numeric/decimal, and integer implementation issues require no further public-policy decisions.