feat(asm): pool the arm64 offsets the split bands cannot carry

Offsets beyond the split bands ride a per-function literal pool the way
the toolchain lays one out: a PC-relative literal load into REGTMP, then
the register-offset access (the pair family adds the base addition), the
pooled words appended after the last instruction behind the UNDEF guard,
deduplicated by value with the sign- and width-aware load selection.

The same differential pass against the corpus exposed three wrong-code
bugs and fixes them: the logical-immediate period marker rode the wrong
position for every element below 64 bits, so the 32-bit forms encoded a
different constant than written; the plain register operand of an
ADD/SUB against SP took the shifted-register form where the toolchain
uses the extended one with the identity extend, silently truncating
through UXTB; and the AUTIA1716 and AUTIB1716 hint constants were the
PACIA and PACIB encodings.  An offset sweep across every band boundary
now pins all three against the live oracle.

Assisted-by: GLM 5.3 Flash

testdata/verify/atomics_arm64.s

@@ -69,6 +69,7 @@ TEXT ·pairs(SB), NOSPLIT, $0-0
 	FLDPD (R8), (F1, F2)
 	FLDPD 8(R8), (F3, F4)
 	FSTPD (F3, F4),-8(R9)
+	FLDPS 4(R8), (F1, F2)
 	FLDPQ 16(R8), (F1, F2)
 	FSTPQ (F1, F2), 48(R9)
 	RET