feat(schema): model enum renames and replace an enum on its name

[omer-cengel]

Summary

Migration histories that rename an enum type, rename an enum label, or replace an enum by declaring a new type under its name now load. Previously every ALTER TYPE action other than ADD VALUE was rejected, and a CREATE TYPE onto an existing enum name failed as Type already exists, so the common replace-an-enum migration could not be read.

Changes

• ALTER TYPE ... RENAME TO renames a modeled enum in its position, keeping its labels; every column of that type, scalar and array alike, follows the new name.
• ALTER TYPE ... RENAME VALUE renames a label in its position, preserving PostgreSQL's sort order; a missing label or an existing new label fails as PostgreSQL rejects it.
• A CREATE TYPE ... AS ENUM or RENAME TO onto a modeled enum's name replaces that enum in its position, because the DROP TYPE that freed the name is not readable. Both usual migration shapes load: renaming the old type away first, or declaring the new type beside it and renaming it onto the old name.
• OWNER TO and the composite attribute actions are ignored without resolving the type. RENAME TO and SET SCHEMA of a type sqlcj does not model are ignored.
• SET SCHEMA of a modeled enum is rejected as Unsupported ALTER TYPE action: SET SCHEMA <schema> at line <n>.
• Schema-parser tests cover each action and both replacement variants. A PostgreSQL integration test applies a replacement migration and runs generated code over the replaced enum and enum-array columns.
• The PostgreSQL guide documents each ALTER TYPE form, the replacement rule, and the remaining diagnostics.

Scope and non-goals

• Type names are still the statement's literal name with delimiters removed, so public.mood and mood are different names. A qualified RENAME TO or SET SCHEMA name that matches no modeled enum fails as Type not found rather than being ignored.
• A qualified RENAME TO target, several comma-joined actions, and SET (...) stay rejected as unreadable type DDL.
• DROP TYPE is still ignored. A column that still names a replaced enum is modeled with the replacing enum's labels, the same unseen DROP TYPE ... CASCADE case already documented.
• Composite types are not modeled.