feat: defmt-over-RTT debug build + probe-rs flow

Add an opt-in `defmt` feature that swaps panic-reset for panic-probe,
links defmt-rtt, and enables hsmc's `trace-defmt` so chart transitions
print over RTT. Default builds are unchanged (still panic-reset, no
defmt overhead).

- Cargo.toml: `defmt` feature pulls defmt + defmt-rtt + panic-probe
  and forwards `trace-defmt` to hsmc; mirror-flag pattern matches
  the existing `embassy` feature.
- build.rs: link `defmt.x` only when the feature is on.
- src/main.rs: cfg-gate the panic handler swap, log boot, USB
  serial, kbd/mouse wake completion vs. deadline, and jiggle dx/dy.
- main.rs: watchdog.pause_on_debug(true) so probe-rs halts don't
  trip the 8 s reset.
- justfile: build-debug, debug, attach, probes recipes
  (release mode — debug builds overflow flash with defmt+panic-probe).
- mise.toml: add cargo:probe-rs-tools.

mise.toml

@@ -7,6 +7,7 @@ just = "latest"
 "cargo:cargo-watch"    = "latest"
 "cargo:cargo-bloat"    = "latest"
 "cargo:cargo-expand"   = "latest"
+"cargo:probe-rs-tools" = "latest"
 
 [env]
 CARGO_TARGET_DIR = "target"