feat(arm64): whole-vector moves, bookkeeping ops and truncating-move lowering

Assisted-by: GLM 5.3 Flash

testdata/verify/bookkeep_arm64.s

@@ -0,0 +1,31 @@
+// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Differential kernel for the arm64 bookkeeping statements: the funcdata.h
+// pseudo-directives (GO_ARGS, NO_LOCAL_POINTERS, FUNCDATA, PCDATA) contribute
+// no instruction bytes, and every function is byte-compared against
+// go tool asm.
+
+#include "textflag.h"
+#include "funcdata.h"
+
+// func bookkeep()
+TEXT ·bookkeep(SB), NOSPLIT, $8-0
+	GO_ARGS
+	FUNCDATA $3, inline_tree(SB)
+	PCDATA   $1, $2
+	MOVD     R1, 0(RSP)
+	RET
+
+// func bookkeepNoLocals()
+TEXT ·bookkeepNoLocals(SB), NOSPLIT, $16-0
+	NO_LOCAL_POINTERS
+	PCDATA            $0, $0
+	PCDATA            $1, $1
+	MOVD              R2, 8(RSP)
+	RET
+
+// func bookkeepPlain()
+TEXT ·bookkeepPlain(SB), NOSPLIT, $0-0
+	MOVD R3, R4
+	RET