feat: report the panic site over MIDI

[kosmar]

Draft: needs a decision against #632 first — see "Interaction with #632" below.

When Core 1 aborts, the device still looks healthy. Core 0 keeps USB enumerated
and the MIDI clock ticking, so the only symptom is that app notes stop. Without
a debug probe attached there is no way to find out where it died.

This replaces panic-probe with a small handler that logs the panic site over
defmt (as before, minus the message text) and records file/line for a Core-0
task, which re-sends them as CCs on MIDI channel 16 once per second. A plain MIDI
capture then carries the panic site. scripts/decode-panic-beacon.py maps the
file hash back to a path.

This is how the Core-1 abort behind the recent "app goes silent after a few
notes" reports was located: the beacon pointed straight at the offending line.

Size: 951,900 bytes of flash vs 949,408 on main, so +2,492 bytes. Dropping
panic-probe saves about 20 KiB; keeping a defmt line for probe users spends
most of it back.

Interaction with #632

#632 sets panic = "immediate-abort", which compiles the #[panic_handler]
away and makes this beacon inert. The two are not textually in conflict (this
branch does not touch Cargo.toml or .cargo/config.toml), but they pull in
opposite directions and want sequencing:

• If fix: support current Rust toolchains #632 lands first, this needs panic = "abort" to do anything, which gives
  back the flash fix: support current Rust toolchains #632 saves.
• If this lands first, fix: support current Rust toolchains #632 silently disables it.

Worth deciding whether post-mortem visibility without a probe is worth a
non-immediate abort. Happy to close this if the answer is no.

Note: cargo clippy on current nightly fails on midi.rs:520 (chunks_exact)
on main as well; #632 fixes that.

Test plan

• Flash and confirm normal operation is unchanged (apps, MIDI, clock).
• Provoke a panic in an app on Core 1, then capture MIDI and confirm CC 110-115
  arrive on channel 16 once per second.
• Run scripts/decode-panic-beacon.py against the capture and confirm it prints
  the correct file and line.
• With a probe attached, confirm the panic site still shows up over RTT.

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