Align status decoding with the manual

Decodes the model code (byte 4) to a name, adds the cooling notification
values, and rewords the error-information-2 bit 0 message. That bit is a
media mismatch, which in practice means a monochrome job sent to a black/red
roll — the exact failure seen on the QL-820NWB here.

src/printing/ql-status.js

@@ -1,23 +1,26 @@
 /**
  * Decoder for the 32-byte status frame the QL series sends back.
  *
- * This is what the Python brother_ql path could never give us: real printer
+ * Field layout and every code below is from Brother's "Raster Command
- * state rather than a parsed traceback. Cover open, roll empty and wrong roll
+ * Reference, QL-800/810W/820NWB v1.01", section 4, "Status information
- * width can all be reported before a label is wasted.
+ * request".
+ *
+ * Note this is only useful over USB. Manual section 5.9 shows the network
+ * flow: over a TCP/IP port the print data is simply sent as-is and no status
+ * comes back. See probeStatus in ql-transport.js.
  *
- * Layout (confirmed against Brother's reference and live QL-8xx captures):
  *   0     print head mark (0x80)
  *   1     size (0x20)
  *   2     fixed 'B' (0x42)
- *   3-4   device dependent (model code)
+ *   3     series code, fixed '4' (0x34)
- *   5     fixed '0' (0x30)
+ *   4     model code
- *   6-7   fixed
+ *   5-7   fixed
  *   8     error information 1
  *   9     error information 2
  *   10    media width in mm
  *   11    media type
  *   12-13 fixed
- *   14    reserved
+ *   14    fixed 0x3F
  *   15    mode
  *   16    fixed
  *   17    media length in mm
@@ -41,13 +44,17 @@ const ERROR_BITS_1 = [
 ];
 
 const ERROR_BITS_2 = [
-  [0x01, 'Wrong media for this job'],
+  [
+    0x01,
+    'Media mismatch — the loaded roll does not match the job. A black/red ' +
+      'roll rejects a monochrome job, and vice versa.',
+  ],
   [0x02, 'Expansion buffer full'],
-  [0x04, 'Transmission or communication error'],
+  [0x04, 'Communication error'],
   [0x08, 'Communication buffer full'],
   [0x10, 'Cover is open'],
   [0x20, 'Cancelled at the printer'],
-  [0x40, 'Media cannot be fed'],
+  [0x40, 'Media cannot be fed, or the end of the media was detected'],
   [0x80, 'System error'],
 ];
 
@@ -65,6 +72,20 @@ const PHASE_TYPES = {
   0x01: 'printing',
 };
 
+/** Notification numbers. Cooling pauses printing but is not a failure. */
+const NOTIFICATIONS = {
+  0x00: null,
+  0x03: 'Print head cooling (started)',
+  0x04: 'Print head cooling (finished)',
+};
+
+/** Byte 4 of the status frame. */
+const MODEL_CODES = {
+  0x38: 'QL-800',
+  0x39: 'QL-810W',
+  0x41: 'QL-820NWB',
+};
+
 const MEDIA_TYPES = {
   0x00: 'none',
   0x0a: 'continuous',
@@ -94,7 +115,7 @@ function decodeStatus(buf) {
   const statusType = STATUS_TYPES[buf[18]] || `unknown_0x${buf[18].toString(16)}`;
 
   return {
-    modelCode: buf.readUInt16BE(3),
+    model: MODEL_CODES[buf[4]] || `unknown_0x${buf[4].toString(16)}`,
     errors,
     hasError: errors.length > 0 || buf[18] === 0x02,
     mediaWidthMm: buf[10],
@@ -104,7 +125,8 @@ function decodeStatus(buf) {
     statusType,
     phaseType: PHASE_TYPES[buf[19]] || `unknown_0x${buf[19].toString(16)}`,
     phaseNumber: buf.readUInt16BE(20),
-    notification: buf[22],
+    notification: NOTIFICATIONS[buf[22]] ?? `unknown_0x${buf[22].toString(16)}`,
+    cooling: buf[22] === 0x03,
     raw: Buffer.from(buf.subarray(0, FRAME_LENGTH)),
   };
 }
@@ -140,8 +162,10 @@ function describeProblem(status) {
 /**
  * Check the loaded roll matches what the job expects.
  *
- * Worth doing before sending: a two-colour job on a plain roll is refused with
+ * Worth doing before sending where status is available: a monochrome job on a
- * "wrong roll type", which gives nobody a clue that a colour setting caused it.
+ * black/red roll is refused outright, and the printer's own message ("change
+ * it to Monochrome media") points at the driver rather than at the roll
+ * setting, which sends people the wrong way.
  *
  * @returns {string|null} a problem description, or null if it matches.
  */
@@ -163,6 +187,7 @@ function checkMediaMatches(status, media) {
 
 export {
   FRAME_LENGTH,
+  MODEL_CODES,
   decodeStatus,
   decodeAll,
   describeProblem,