UnsupportedDirection

Marks an overridden convertTo/convertFrom STUB as intentionally unsupported, with a compile-time guiding reason. The stub body must be = unsupported(). Annotate the TOTAL method only (annotating an OrNull variant is a compile error). The annotation wins over the body: an annotated direction is treated as unsupported by the compiler regardless of body.

Retention is BINARY (not SOURCE) on purpose: converters usually live in a DIFFERENT module than the mapping they serve (built-ins always do), so the consumer's KSP round resolves them from the classpath — a SOURCE-retained annotation would be invisible there and the direction would silently look provided. BINARY keeps the declaration readable cross-module while staying invisible to runtime reflection.