Function fn_key_for_decl 

pub fn fn_key_for_decl(ctx: &CodegenContext, fd: &FnDef) -> FnKey

Round-7: build a crate::ir::FnKey from a borrowed &FnDef. The owning module is resolved by pointer-comparison against ctx.modules[*].fn_defs (entry items / synthesized variants / extra_fn_defs fall through to FnKey::entry). This is the single resolver consumers should reach for from emit code with a &FnDef in hand — produces the typed key the IR maps store instead of leaving the bare-name collision risk in place.