fix(parser): read the segment-absolute rendering FS:0

The toolchain's disassembler prints the segment-prefixed disp32
absolute as FS:0, but the bare-name branch read the segment register
alone and dropped the offset, so MOVQ FS:0, DX silently encoded a
register move.  The colon-offset form now lowers to the same
segment-absolute operand the 0(FS) spelling takes.

Assisted-by: GLM 5.3 Flash

parser/parser.go

@@ -678,6 +678,21 @@ func parseAddress(g []token.Token) ast.Address {
 			i++
 		}
 	}
+	// Segment-absolute in the rendering spelling: FS:0 and GS:0.  The
+	// colon-offset form is what the toolchain's disassembler prints for the
+	// segment-prefixed disp32 absolute, so the rendered text must re-encode;
+	// without it the segment register alone parsed and the offset vanished.
+	// The base field naming the segment lowers to the same operand the
+	// 0(FS) spelling takes.
+	if len(g) >= 3 && g[0].Kind == token.Ident && g[1].Kind == token.Colon &&
+		(g[0].Text == "FS" || g[0].Text == "GS") {
+		if v, rest, ok := foldExpr(g[2:]); ok && len(rest) == 0 {
+			addr.Base = g[0].Text
+			addr.Offset = v
+			addr.HasOff = true
+			return addr
+		}
+	}
 	// Bare name (register, label or symbol) possibly with an arm64 shift.
 	if addr.Base == "" && addr.Sym == nil && g[0].Kind == token.Ident {
 		sym := &ast.Symbol{Pos: g[0].Pos}