fix(lexer): tokenise the flag separator and handle NUL and invalid UTF-8

Assisted-by: GLM 5.3

asm/guard_test.go

@@ -6,6 +6,7 @@ package asm
 import (
 	"bytes"
 	"encoding/hex"
+	"strings"
 	"testing"
 
 	"sourcedock.dev/petrbalvin/gasm-devkit/ast"
@@ -27,6 +28,12 @@ func TestStackGuardBytes(t *testing.T) {
 			"644c8b3425000000004c8da42478ffffff4d3b66107614554889e54881ec000100004881c4000100005dc3e800000000ebce"},
 		{"leafbig", "TEXT \u00b7leafbig(SB), $8192-0\n\tRET\n",
 			"644c8b3425000000004989e44981ec881f0000721a4d3b66107614554889e54881ec002000004881c4002000005dc3e800000000ebca"},
+		// Class 2 with a body long enough that the underflow JB relaxes to
+		// rel32: its displacement must span the real 6-byte JB, else the
+		// branch lands 4 bytes past the morestack block, inside the CALL
+		// displacement field.
+		{"leafbiglong", "TEXT \u00b7leafbiglong(SB), $8192-0\n" + strings.Repeat("\tMOVQ AX, BX\n", 40) + "\tRET\n",
+			"644c8b3425000000004989e44981ec881f00000f82960000004d3b66100f868c000000554889e54881ec00200000" + strings.Repeat("4889c3", 40) + "4881c4002000005dc3e800000000e947ffffff"},
 		{"callsmall", "TEXT \u00b7callsmall(SB), $16-0\n\tCALL \u00b7other(SB)\n\tRET\nTEXT \u00b7other(SB), NOSPLIT, $0\n\tRET\n",
 			"644c8b342500000000493b66107613554889e54883ec10e8000000004883c4105dc3e800000000ebd7"},
 		{"nosplit", "TEXT \u00b7nosplit(SB), NOSPLIT, $16-0\n\tRET\n",