…erence

The existing cases either bind to a typed local (SAM TInst lands in the
AST directly) or call a static method (TCall callee's TField has a TFun
etype that exposes parameter types to the scan). Neither exercises the
path that AbstractCast's functional_interfaces_used writeback is needed
for: a bound instance-method reference passed to a Java extern's
constructor, with the SAM type never named anywhere else in user code.

Add CtorOnly + CtorSam to test.Listeners and a Holder class in Main.hx
that does `new Listeners_CtorSam(onCtor, 42)`. Without the writeback the
emitted closure does not implement CtorOnly and the JVM throws
ClassCastException at runtime — same shape as RideAssist's
`new TextToSpeech(this, onTtsInit)` crash against android.jar.

…e in AST

Replace the functional_interfaces_used hashtbl plumbing (Common.context
+ Gctx.t + AbstractCast writeback + genjvm seeding) with a one-line
change in AbstractCast: wrap the function expression in mk_cast — a
TCast(eright, None) with the SAM type as the cast's etype.

The SAM TInst now lives in the typed AST at every conversion site, so
genjvm's existing collect_used_functional_interfaces scan finds it
naturally via note_fi_in_type on the TCast's etype. No cross-module
mutable state, no typing↔codegen back-channel, no @:native path-rewrite
bridging.

mk_cast is already the established pattern in this file — it's what the
MultiType abstract path uses a few lines up.