w6a: MOVB REX for SI/DI/BP/SP; LEAQ/CALL shapes err; honest diag lengths

Three w6a defects, one component. MOVB with an SP/BP/SI/DI register
operand encoded AH/CH/DH/BH -- emit_rex suppressed the bare 0x40
that selects SPL/BPL/SIL/DIL (the comment claimed "we always emit
REX"; encode probe: `MOVB SI,(AX)` -> `88 30` = %dh). emit_rex8
forces the byte for low-byte codes 4-7; no current w6c output emits
those shapes, so all existing objects are unchanged. LEAQ and CALL
unsupported operand shapes fell through with zero bytes and no
errs++ (every MOV arm reports); both now err. Every hardcoded
os.write diagnostic length in the wwstage was one byte short
(truncating the newline/quote); all converted to the .len idiom so
the length cannot drift again.

selfhost/cmd/w6a/main.ww

@@ -73,16 +73,19 @@ export fn main(argc: i32, argv: **u8) i32 = {
 		if (cstreq(a, "-o")) {
 			i += 1;
 			if (i >= argc) {
-				os.write(2, "w6a: -o requires arg\n".ptr, 20u64);
+				let wm1: str = "w6a: -o requires arg\n";
+				os.write(2, wm1.ptr, wm1.len: u64);
 				return 2;
 			};
 			out = argv[i];
 		} else { if (a[0u64] == 45u8) {
-			os.write(2, "w6a: unknown flag\n".ptr, 17u64);
+			let wm2: str = "w6a: unknown flag\n";
+			os.write(2, wm2.ptr, wm2.len: u64);
 			return 2;
 		} else {
 			if (src != nil) {
-				os.write(2, "w6a: only one input\n".ptr, 19u64);
+				let wm3: str = "w6a: only one input\n";
+				os.write(2, wm3.ptr, wm3.len: u64);
 				return 2;
 			};
 			src = a;
@@ -91,11 +94,13 @@ export fn main(argc: i32, argv: **u8) i32 = {
 	};
 
 	if (src == nil) {
-		os.write(2, "usage: w6a_ww -o file.o file.s\n".ptr, 30u64);
+		let wm4: str = "usage: w6a_ww -o file.o file.s\n";
+		os.write(2, wm4.ptr, wm4.len: u64);
 		return 2;
 	};
 	if (out == nil) {
-		os.write(2, "w6a: missing -o\n".ptr, 15u64);
+		let wm5: str = "w6a: missing -o\n";
+		os.write(2, wm5.ptr, wm5.len: u64);
 		return 2;
 	};
 
@@ -103,7 +108,8 @@ export fn main(argc: i32, argv: **u8) i32 = {
 	let blen: u64;
 	buf, blen = slurp(src);
 	if (buf == nil) {
-		os.write(2, "w6a: cannot read input\n".ptr, 22u64);
+		let wm6: str = "w6a: cannot read input\n";
+		os.write(2, wm6.ptr, wm6.len: u64);
 		return 1;
 	};
 
@@ -120,7 +126,8 @@ export fn main(argc: i32, argv: **u8) i32 = {
 
 	let fd: i32 = os.open(pathstr(out), os.flag.WRONLY | os.flag.CREATE | os.flag.TRUNC, 420i32);	// 0o644
 	if (fd < 0) {
-		os.write(2, "w6a: cannot open output\n".ptr, 23u64);
+		let wm7: str = "w6a: cannot open output\n";
+		os.write(2, wm7.ptr, wm7.len: u64);
 		return 1;
 	};
 	let rc: i32 = emitelf(&s, fd);