refactor: split 832-line main.rs into focused modules

config (constants), usb (driver + tasks), led (NeoPixel anims), mouse
(cursor anims + HID), kbd (F13 wake + HID), chart (statechart + Ctx/Ev
+ JigglyActions). main.rs is now just Irqs + USB builder + chart.run().

No behavior change — comments, deadlines, and tunables preserved verbatim.
Builds clean on default and --features embassy,defmt; clippy -D warnings
passes.

src/usb.rs

@@ -0,0 +1,57 @@
+//! USB driver type aliases, identity helpers, and infra tasks
+//! (USB device pump + watchdog feeder).
+
+use embassy_rp::{
+    Peri,
+    flash::Flash,
+    peripherals::{FLASH, USB},
+    usb::Driver,
+    watchdog::Watchdog,
+};
+use embassy_time::Timer;
+use embassy_usb::UsbDevice;
+use static_cell::StaticCell;
+
+use crate::config::{WATCHDOG_FEED_INTERVAL, WATCHDOG_TIMEOUT};
+
+pub(crate) type UsbDriver = Driver<'static, USB>;
+
+// ── USB serial number from RP2040 unique chip ID ───────────────────
+// Reads the 64-bit unique ID baked into the on-board SPI flash, formats
+// it as 16 ASCII hex chars in a static buffer, and returns a `'static`
+// string suitable for `embassy_usb::Config::serial_number`. This makes
+// the device's USB identity stable per-board across replugs (so hosts
+// stop treating each plug as a new device) while still being unique
+// between different boards.
+const FLASH_SIZE: usize = 2 * 1024 * 1024; // Xiao RP2040 has 2 MB.
+
+pub(crate) fn make_serial(flash_periph: Peri<'static, FLASH>) -> &'static str {
+    static SERIAL: StaticCell<[u8; 16]> = StaticCell::new();
+    const HEX: &[u8; 16] = b"0123456789ABCDEF";
+
+    let mut flash = Flash::<_, _, FLASH_SIZE>::new_blocking(flash_periph);
+    let mut id = [0u8; 8];
+    let _ = flash.blocking_unique_id(&mut id);
+
+    let buf = SERIAL.init([0; 16]);
+    for (i, &b) in id.iter().enumerate() {
+        buf[i * 2] = HEX[(b >> 4) as usize];
+        buf[i * 2 + 1] = HEX[(b & 0x0f) as usize];
+    }
+    core::str::from_utf8(buf).unwrap()
+}
+
+// ── Tasks ──────────────────────────────────────────────────────────
+
+#[embassy_executor::task]
+pub(crate) async fn usb_task(mut device: UsbDevice<'static, UsbDriver>) {
+    device.run().await;
+}
+
+#[embassy_executor::task]
+pub(crate) async fn watchdog_task(mut wd: Watchdog) -> ! {
+    loop {
+        wd.feed(WATCHDOG_TIMEOUT);
+        Timer::after(WATCHDOG_FEED_INTERVAL).await;
+    }
+}