feat: initial Xiao RP2040 mouse jiggler firmware

Embassy-based no_std firmware for the Seeed Studio Xiao RP2040 that
masquerades as a Dell MS116 USB HID Boot Mouse. Each cycle picks a
random axis from the RP2040 ROSC RANDOMBIT register, nudges +1 px,
dwells 25 ms, nudges -1 px, then sleeps 4.5 minutes. After 8 hours
elapsed from boot or RESET press the device idles silently (watchdog
still fed) until the user taps RESET (R) again, aligning a single
press with one workday.

Tooling:
- mise.toml pins rust 1.95.0 with explicit components and the
  thumbv6m-none-eabi target, plus cargo-binutils / uf2conv helpers.
- justfile shell is set to "mise exec -- sh -eu -c" so every recipe
  uses the pinned toolchain without any shell activation.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

build.rs

@@ -0,0 +1,12 @@
+use std::{env, fs::File, io::Write, path::PathBuf};
+
+fn main() {
+    let out = PathBuf::from(env::var_os("OUT_DIR").unwrap());
+    File::create(out.join("memory.x"))
+        .unwrap()
+        .write_all(include_bytes!("memory.x"))
+        .unwrap();
+    println!("cargo:rustc-link-search={}", out.display());
+    println!("cargo:rerun-if-changed=memory.x");
+    println!("cargo:rerun-if-changed=build.rs");
+}