fix(schemas): add validation for child schemas in discriminated unions

[leoafarias]

Discriminated unions require child schemas to return Map types. This commit
adds proper runtime validation with a helpful error message when a child
schema returns a non-Map type (e.g., when using .transform() on a
child schema).

The error message guides users to the correct pattern: apply .transform()
to the discriminated union itself, not to individual child schemas.

Changes:

• Added runtime type check after child schema validation
• Added helpful error message explaining the correct transform pattern
• Added documentation with code example showing correct usage
• Added test verifying the error message and correct pattern

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[leoafarias]

Closing as superseded by the later discriminated-union contract. Current Ack intentionally supports object-backed transformed child branches and tests parse, safeParse, validation ordering, transform failures, and multi-layer transforms. This PR would restore an obsolete rejection of those branches.