feat(schema/mysql): add integer unsigned and mode options

[scottescue]

Outcome

MySQL int, bigint, mediumint, smallint, and tinyint builders expose correct unsigned behavior and the documented bigint mode mapping.

Context

Several builders are re-exported from PostgreSQL or implement only signed values, so the current DSL cannot represent MySQL unsigned DDL and generated Go types consistently.

In scope

• Unsigned options for int, bigint, mediumint, smallint, and tinyint.
• Bigint value-mode mapping.
• Snapshot representation, generated signed/unsigned Go types, and deterministic MySQL DDL.
• Range/default validation where signedness changes accepted values.

Out of scope

• Serial/autoincrement redesign.
• Floating, decimal, or temporal options.
• Driver codec work beyond the selected schema value representation.

Acceptance criteria

• Every named integer builder exposes the documented typed unsigned option; bigint also exposes the ratified mode.
• Signed/unsigned defaults and ranges validate before snapshot or SQL generation.
• Snapshots and generated Go types preserve signedness/mode deterministically.
• MySQL DDL renders unsigned in the normative position and leaves signed output unchanged.
• Tests cover each builder, signed/unsigned boundaries, bigint modes, invalid values, and round trips.