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>

mise.toml

@@ -0,0 +1,16 @@
+[tools]
+rust = { version = "1.95.0", profile = "minimal", components = ["cargo", "clippy", "llvm-tools", "rust-src", "rust-std", "rustc", "rustfmt"], targets = ["thumbv6m-none-eabi"] }
+just = "latest"
+
+"cargo:cargo-binutils" = "latest"
+"cargo:uf2conv"        = "latest"
+"cargo:cargo-watch"    = "latest"
+"cargo:cargo-bloat"    = "latest"
+"cargo:cargo-expand"   = "latest"
+
+[env]
+CARGO_TARGET_DIR = "target"
+DEFMT_LOG        = "off"
+
+[settings]
+experimental = true