fix pale colours and map driverless print options

Set *DefaultBRColorMode: None in the vendor PPD so every job sends
<</UseCIEColor false>>: Brother's Normal/Vivid modes enable the printer's
own CIE conversion, which washed the colours out.

Add six APPrinterPreset entries mapping print-color-mode and
print-quality from driverless (AirPrint/IPP) clients onto the Brother
device options; they use the notation CUPS generates itself (Apple
metadata as 'key value' pairs, PPD options as '*Name Value'), because
key=value strings are silently discarded. print-quality is 3/4/5.

Add the PRINTER_OPTIONS env var: whitespace-separated key=value pairs
applied as queue defaults on every start via lpadmin -o, validated
against the queue's PPD, with warnings instead of a failed startup.

Document all of it in the README. The VividNoCIE candidate was dropped
after test prints showed no difference from None. Archive the OpenSpec
change and sync the printserver/cups spec with the two new
requirements.

README.md

@@ -85,11 +85,26 @@ podman run -d --name printserver \
 | `PRINTER_NAME` | `HL4150CDN` | CUPS queue name. Also used as the name advertised via AirPrint/DNS-SD. |
 | `PRINTER_DESCRIPTION` | *(empty)* | CUPS printer description (`lpadmin -D`), shown in the CUPS web UI. |
 | `PPD_FILE` | `/usr/share/cups/model/BR4150_2.PPD` | PPD used for the queue. |
+| `PRINTER_OPTIONS` | *(empty)* | Space-separated PPD option assignments (`key=value`) applied as queue defaults on every start, e.g. `BRColorMode=Vivid TonerSaveMode=False`. Discover the names with `lpoptions -p <queue> -l`. |
 | `CERT_EXTRA_NAMES` | *(empty)* | Comma-separated extra DNS names and/or IP addresses to put into the generated `_ipps` certificate. |
 | `CERT_DAYS` | `3650` | Validity of the generated certificate, in days. |
 
 No queue is created when neither `PRINTER_IP` nor `PRINTER_URI` is set.
 
+`PRINTER_OPTIONS` sets queue defaults that survive the recreation of the queue,
+because they are re-applied on every start:
+
+```sh
+podman run -d --name printserver \
+  --network host \
+  -e PRINTER_IP=192.168.100.7 \
+  -e PRINTER_OPTIONS="BRMediaType=Plain BRColorMode=Vivid TonerSaveMode=False" \
+  printserver:latest
+```
+
+Entries that the PPD does not know are logged as a warning and ignored; the
+remaining ones are applied and the container starts normally.
+
 ### Choosing the connection
 
 The HL-4150CDN accepts PostScript directly, so the default is raw AppSocket.
@@ -187,7 +202,7 @@ The BR-Script3 PPD exposes the HL-4150CDN's hardware options. Common ones:
 | Paper source | `InputSlot` | `AutoSelect`, `Tray1`, `Tray2`, `MPTray` |
 | Two-sided printing | `Duplex` | `None`, `DuplexNoTumble` (long edge), `DuplexTumble` (short edge) |
 | Colour / mono | `BRPrintQuality` | `Auto`, `Color`, `Black` |
-| Colour mode | `BRColorMode` | `Normal`, `Vivid`, `None` |
+| Colour mode | `BRColorMode` | `Normal`, `Vivid`, `None` (default) |
 | Media type | `BRMediaType` | `Plain`, `Thin`, `Thick`, `Recycled`, `Glossy`, `Label`, envelopes, … |
 | Print quality | `CAPT` | `Fine` (600×600), `SuperFine` (2400 dpi class) |
 | Toner save | `TonerSaveMode` | `True`, `False` |
@@ -200,6 +215,57 @@ podman exec printserver lp -d HL4150CDN \
   -o Duplex=DuplexNoTumble -o BRPrintQuality=Color -o InputSlot=Tray1 report.pdf
 ```
 
+### Why the macOS/iOS dialog shows fewer options
+
+macOS and iOS print driverless (AirPrint / IPP Everywhere), so the dialog shows
+only IPP-standard options: media size, duplex, pages per sheet, the
+black-and-white/colour toggle and the quality slider. Brother-specific options
+(`BRColorMode`, `BRMediaType`, `TonerSaveMode`, `CAPT`, …) have no IPP
+equivalent and can never appear there. Set them server-side instead:
+
+- as queue defaults at container start via `PRINTER_OPTIONS` (see
+  *Environment variables*),
+- per job with `lp -o …`, or
+- interactively in the CUPS web UI (`http://<host>:631/admin`, *Printing Options*).
+
+The dialog's black-and-white/colour toggle maps to `print-color-mode` and the
+quality slider to `print-quality`; the PPD presets documented below translate
+both onto the Brother options, so they now really take effect:
+
+| Dialog / IPP | PPD option it becomes |
+| --- | --- |
+| `print-color-mode=monochrome` | `BRPrintQuality=Black` |
+| `print-color-mode=color` | `BRPrintQuality=Auto` |
+| `print-quality=3` (draft) | `TonerSaveMode=True` |
+| `print-quality=4` (normal) | defaults (`Fine`, toner save off) |
+| `print-quality=5` (high) | `CAPT=SuperFine` (2400 dpi class) |
+
+Client-specified values always win over the preset.
+
+### Changes to the vendor PPD
+
+`assets/Brother HL-4150CDN/ppd/BR4150_2.PPD` is Brother's original file plus
+two additions, each marked with a `%` comment in the file:
+
+- **`*DefaultBRColorMode: None` instead of `Normal`.** `Normal` and `Vivid` both
+  set `<</UseCIEColor true>>`; the printer's own CIE conversion washes the
+  colours out (see [Colours are printed pale / washed out](#colours-are-printed-pale--washed-out)).
+- **Six `APPrinterPreset` entries** mapping `print-color-mode`
+  (color/monochrome) and `print-quality` (draft/normal/high) onto
+  `BRPrintQuality`, `CAPT` and `TonerSaveMode`. CUPS applies such presets for
+  driverless clients and always lets the client's own option values win. In
+  particular `print-color-mode=monochrome` becomes `BRPrintQuality=Black`, so
+  black-and-white printing works: the server cannot do the greyscale conversion
+  itself, because `pdftops` uses Poppler for Brother printers and Poppler
+  cannot produce greyscale (`WARNING: Grayscale/monochrome could be printed as
+  color with Poppler` in the CUPS log). The presets use the notation CUPS
+  generates itself — Apple metadata as `key value` pairs *without* `=`, PPD
+  options as `*Name Value` pairs on separate lines inside a multiline
+  `"…"` string. With `key=value` on a single line CUPS silently ignores the
+  whole preset (verified against libcups 2.4.2: `_ppdParseOptions()` splits on
+  whitespace, so `key=value` becomes the option *name* and the lookup fails).
+  Note that IPP `print-quality` is `3` (draft), `4` (normal), `5` (high).
+
 ## Client support
 
 | Platform | Path | Notes |
@@ -384,6 +450,38 @@ still being rejected — fix that, or add the queue from the command line as
 above. macOS caches the failed attempt, so remove and re-add the printer after
 changing anything on the server.
 
+### Colours are printed pale / washed out
+
+Symptom: colour printouts look desaturated compared to the same PDF printed
+directly from the Mac, or with Brother's own driver.
+
+Cause: the PPD colour mode `Normal` (Brother's default) makes the printer's
+PostScript interpreter apply its own CIE conversion (`<</UseCIEColor true>>`),
+which washes colours out. `Vivid` sets `UseCIEColor` as well, so it does not
+help either; `BRPrintQuality` (`BRColorAdapt`) and `TonerSaveMode` made no
+visible difference in a side-by-side test. The container therefore ships the
+PPD with `*DefaultBRColorMode: None` (`UseCIEColor` off), which produces
+saturated output.
+
+Check and, if needed, override the current default:
+
+```sh
+podman exec printserver lpoptions -p <queue> -l | grep BRColorMode   # expect *None
+podman exec printserver lp -d <queue> -o BRColorMode=None test.pdf
+```
+
+If output is still pale with `BRColorMode=None`, check the printer's own menu
+(Toner → Toner Save off) and make sure `BRMediaType` matches the media in the
+tray.
+
+Black-and-white is a related but separate path: `print-color-mode=monochrome`
+from a driverless client is mapped by the PPD presets onto
+`BRPrintQuality=Black` (the printer renders mono). It is deliberately *not* done
+by the server — with its Poppler renderer, `pdftops` cannot convert to
+greyscale. Forcing Ghostscript with `-o pdftops-renderer=gs` would, but that
+path is not recommended for Brother printers (bugs in their PostScript
+interpreters).
+
 ## CUPS 3 and the future of PPDs
 
 Adding the queue with a PPD makes `lpadmin` print: