STM32F746G-DISCO: the car HMI demo out of the box

[JaxCalder]

Summary

After this, cmake -DBOARD=stm32f746g-disco → build → install-all gives a board running the car HMI demo, with the MP3 player for the card's music/:

• configs/board/stm32f746g-disco/flash.dmd:
  • dmaudio service and rules: one dmaudio@wm8994 for the codec
  • dmview@>=0.10 with its service and rules: the claim channel carhmi needs. The version is on every line, because a line without it pulls dmview/libdmview 0.9 from the registry and overwrites the 0.10 the other line installed.
  • carhmi service=carhmi.ini
• sdcard.dmd: carhmi_view (≈320 kB, goes to build/sdcard/views/carhmi_view/).
• configs/common/flash.dmd: ymodem. upload-files / install-all type ymodem recv, but no image contained the command, so on a fresh clone the upload could not work.
• lib/dmod → Dmod_Run: do not hold the module's mutex while its main() runs dmod#330: Dmod_Run() no longer holds the module's mutex for the whole main(). With it held, every service ran permanently at systemd's priority 4, the drawing of the view starved the MP3 decoder, and playback crackled.
• README: a section on the demo, including how to put music on the card.

Flash: 686 KB of 1 MB used, 338 KB free.

Before merging

Released and in the registry first:

• Dmod_Run: do not hold the module's mutex while its main() runs dmod#330 (then point lib/dmod at the merged develop commit)
• The channel of a claim: view properties, .change handlers, EMIT (format 0.6) dmview#18 (0.10) with todmv#8
• dmmp3: MP3 files - tags, length, decoding, seeking (minimp3) dmmp3#1, carhmi-demo#1 + Add CMake build system with full Make parity #2, dmod-registry#14 (registers dmmp3 and carhmi-demo)
• for carhmi_view's conversion: dmvsi#7, todmvs#5, dmvs_js#5, dmvs_html#5

Until then CI can't resolve carhmi, carhmi_view, or dmview@>=0.10.

Test plan

• Fresh worktree of origin/master + this branch, built with a local manifest standing in for the unreleased packages: the image contains dmview/libdmview 0.10, carhmi, dmmp3, libdmaudio/dmaudio 0.1, ymodem 0.14, and the configs of carhmi, dmview, dmaudio and the codec
• STM32F746G-DISCO: install-all flashes and sends the view (62 files, 16.7 s). carhmi shows the card's tracks with their ID3 tags, and playback is clean with the dmod fix.
• CI after the releases

Known issues (not in this PR)

• Large uploads (>1 MB) are cancelled by the device partway through (seen twice: at ≈1.2 MB and ≈2.4 MB). Smaller files (the view, MP3s up to 400 KB) go through.
• dmfatfs can lose files written shortly before a reset, so give it a few seconds after an upload.

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