Retire the @asConnection(connectionName:) deprecated argument

Per-field source-location provenance (R206) cannot become an invariant. R206 threads the @asConnection carrier’s SourceLocation through into the synthesised ConnectionType / EdgeType records. R206’s plan §1 documents first-write-wins as the dedupe rule because, with connectionName: still legal, two carriers can legitimately name the same synthesised type. Once the override is gone, a synthesised connection name maps to exactly one carrier and the "carrier-field source location" becomes well-defined; the validator can promote the assumption to a validator-enforced invariant rather than a documented convention. (R206’s plan §1 references that future invariant explicitly.)

Two diagnostic phrasings to maintain. Rejection.java:243-246 carries two parallel fix hints for the case-fold collision arm: a "rename the source field or set @asConnection(connectionName:) to a name that is unique" path and a "rename the connection (source field or @asConnection(connectionName:))" edge-name variant. When the override retires, both messages simplify to the rename-the-source-field path.

Test fixtures use the override to force collisions that no realistic schema produces. GraphitronSchemaBuilderTest.CaseInsensitiveTypeClashCase.SYNTH_VS_SYNTH and SDL_VS_SYNTH use @asConnection(connectionName: "FooConnection") / (connectionName: "fooConnection") to manufacture a case-fold collision between synth names. Without the override, the equivalent collision is engineered via two carriers whose default-derived names differ only in case (e.g. Foo.x vs foo.X, or Query.fooConn vs Query.FooConn). The arm continues to test what it tests, but the SDL fixture stops being "schema-author writes a duplicate `connectionName:`" and becomes "schema-author writes two collision-prone field names".