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.

build.rs

@@ -9,4 +9,11 @@ fn main() {
     println!("cargo:rustc-link-search={}", out.display());
     println!("cargo:rerun-if-changed=memory.x");
     println!("cargo:rerun-if-changed=build.rs");
+
+    // defmt ships its own linker script (`defmt.x`) that defines the
+    // `_defmt_*` interner sections. Only link it when the feature is on —
+    // otherwise the symbols don't exist and `ld` will refuse the build.
+    if env::var_os("CARGO_FEATURE_DEFMT").is_some() {
+        println!("cargo:rustc-link-arg=-Tdefmt.x");
+    }
 }