statement: order DeclarativeToImperative output as CREATE → ALTER → DROP

[morgo]

Summary

Orders the output of DeclarativeToImperative as CREATE → ALTER → DROP instead of the previous behavior where CREATEs and ALTERs were interleaved alphabetically.

Motivation

When a diff produces both CREATE TABLE and ALTER TABLE statements, the ALTER may reference the newly-created table (e.g., adding a foreign key to a new table). By ensuring CREATEs come first, the output is safe to execute sequentially without dependency issues.

Changes

• Split the single changes slice into separate creates and alters slices
• Combined them as creates + alters + drops at the end
• Updated the existing ordering test to verify the three-phase ordering

Fixes #671

[Copilot]

Pull request overview

This PR updates DeclarativeToImperative statement ordering to emit DDL in three deterministic phases—CREATE → ALTER → DROP—to avoid dependency issues when sequentially executing diffs (notably when ALTERs reference newly created tables).

Changes:

• Split accumulated statements into creates, alters, and drops buckets.
• Emit the final result as creates + alters + drops (each bucket already deterministic via name sorting).
• Update the existing ordering test to assert strict three-phase ordering.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
pkg/statement/declarative.go	Changes output ordering to CREATE-first, then ALTER, then DROP.
pkg/statement/declarative_test.go	Updates ordering test to validate CREATE → ALTER → DROP sequencing.

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