bug(writer): DECORATED_BY silently drops every class-level decorator #1601

Split out of #1595 (report 4 of 9) — filed by @rrodriguesNutrium, credit to them.

DECORATED_BY is declared as a two-pair relationship group (database_embedded_kuzu.py:288):

("DECORATED_BY", "FROM Function TO Function, FROM Class TO Function, line_number INT64", True)

But write_decorated_by_links (writer.py:1126-1136) hardcodes :Function on both endpoints, so a decorated class never matches and its edge is dropped with no error and no log. build_decorated_by_links builds those rows correctly — they just never land.

Reproduced on the #1596 fixture: the builder emits 7 links, of which UserViewModel -> HiltViewModel and UserEntity -> Entity are class-decorated and lost at write time.

This affects every language that records class-level decorators, not just Kotlin — Python's @dataclass, TypeScript's @Component, and so on take the same path.

Fix direction: parameterise the source label the way write_inheritance_links already does.

Worth a broader sweep, per the reporter: a relationship declared as a REL TABLE GROUP with multiple FROM..TO pairs but a writer that only queries one pair is a silent-drop pattern. There may be others.