Added the CDJ-900NXS firmware patcher.

Takes the official C900NXS.UPD (1.31, checked by SHA-256), lets you choose
mods in a list (arrows, space, enter; numbers where the terminal cannot pass
keys), patches the application with the emulator checkout's
mods/patch_cdj900nxs.py, packs it and writes a new C900NXS.UPD labelled 1.32.

Only the application is replaced: the boot loader and recovery records,
MAIN's S0/S7 records, DRIV and PANL are copied byte for byte and checked again
on the finished file, which is also unpacked the boot loader's way and
compared with the patched application. --stock differs from the official file
in the version label and its CRC only.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

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+# CDJ-900NXS firmware patcher
+
+Builds a new `C900NXS.UPD` from the official CDJ-900NXS firmware 1.31 with
+the mods you choose, for the player's own USB update.
+
+```sh
+python cdj900nxs_patcher.py C900NXS.UPD                  # choose in a list
+python cdj900nxs_patcher.py C900NXS.UPD --mods beatjump,cuecolours,hotcuekeys
+python cdj900nxs_patcher.py C900NXS.UPD --stock          # the original, to go back
+python cdj900nxs_patcher.py --list
+```
+
+In the list: **arrow keys** move, **space** ticks or unticks, **a** ticks all
+or none, **enter** builds, **q** quits. Terminals that cannot pass single
+keys (Git Bash's mintty, a pipe) get a numbered list instead. The result goes
+to `out/<date>-<mods>/C900NXS.UPD` with a `report.txt` of every check.
+
+## What it guarantees
+
+- **Only the original file is accepted**: SHA-256
+  `e96bca7d1ac983985992a8e5fa7813e1ebf04bef514e0c6cfd0a8375f667f7f9`, and the
+  1.31 application inside it is checked too.
+- **Only the main application is replaced.** The boot loader and recovery
+  loader (MAIN's records below flash 0x40000), MAIN's S0 header and S7 entry
+  records, and the DRIV and PANL sections are copied from the original byte
+  for byte. Before anything is written the finished file is read back and
+  checked: every section CRC and S-record checksum; DRIV and PANL equal to the
+  original; MAIN's header equal but for its version label, application-only
+  mode kept; every boot loader and recovery record and byte equal; no record
+  past 0x7E0000; the packed application's checksum; and the application,
+  unpacked the way the boot loader does it, equal to the patched one.
+- **Labelled 1.32**, so that the player's updater does not skip the update as
+  the version it already has. The application still reports 1.31 (as
+  arnoldasr's builds do). `--stock` gives the original firmware with only that
+  label changed: three bytes differ from the official file (the digit and the
+  section CRC), exactly as arnoldasr's back-to-stock file, which installed on
+  his test player.
+
+Checked in the emulator: both the `--stock` file and one with every mod were
+written into an emulated flash and booted through Pioneer's own boot loader,
+which unpacked and started them.
+
+## Mods
+
+From the emulator checkout `../CDJ900NXSVirtual` (`--repo` for another place):
+`mods/patch_cdj900nxs.py` and the mods next to it.
+
+| mod | what it does |
+|---|---|
+| `code` | arnoldasr: BPM with two decimals, two-line phase meter, RGB waveform |
+| `utility` | arnoldasr: UTILITY > PHASE METER switch (needs `code`) |
+| `overview` | arnoldasr: overview waveform colour gradient |
+| `beatjump` | TRACK SEARCH jumps 1/2/4/8 beats, UTILITY > BEAT JUMP |
+| `cuecolours` | memory cues in their rekordbox colours, hot cues shown |
+| `hotcuekeys` | UTILITY > HOT CUE KEYS; QUANTIZE short = hot key mode |
+
+arnoldasr's mods need his repository checked out in
+`../CDJ900NXSVirtual/mods/cdj900nxs/upstream` (it has no licence, so it is
+not copied here); without it they are greyed out.
+
+## Risk
+
+Nothing here talks to a player; you copy the file to a FAT32 USB stick and run
+the update yourself. Our mods have only run in the emulator. Recovery from an
+application that crashes at start-up has not been tested by anyone. Keep the
+`--stock` file at hand.
+
+## Tests
+
+```sh
+CDJ900NXS_UPD=/path/to/C900NXS.UPD python -m pytest test_patcher.py
+```