fix: gate nusb/hardware discovery to Linux/macOS/Windows only

Android (Termux) reports target_os="android" which is not supported
by nusb::list_devices(). This caused E0425 and E0282 compile errors
when building on Termux.

Changes:
- Cargo.toml: move nusb to a target-gated dependency block so it is
  only compiled on linux/macos/windows
- src/hardware/discover.rs: add #![cfg(...)] file-level gate matching
  the nusb platform support matrix
- src/hardware/mod.rs: gate discover/introspect module declarations,
  discover_hardware() call, handle_command() dispatch, and all helper
  fns on the same platform set; add a clear user-facing message on
  unsupported platforms
- src/security/pairing.rs: replace deprecated rand::thread_rng() with
  rand::rng() to keep clippy -D warnings clean

Fixes #880

Cargo.toml

@@ -130,12 +130,14 @@ opentelemetry = { version = "0.31", default-features = false, features = ["trace
 opentelemetry_sdk = { version = "0.31", default-features = false, features = ["trace", "metrics"] }
 opentelemetry-otlp = { version = "0.31", default-features = false, features = ["trace", "metrics", "http-proto", "reqwest-client", "reqwest-rustls-webpki-roots"] }
 
-# USB device enumeration (hardware discovery)
-nusb = { version = "0.2", default-features = false, optional = true }
-
 # Serial port for peripheral communication (STM32, etc.)
 tokio-serial = { version = "5", default-features = false, optional = true }
 
+# USB device enumeration (hardware discovery) — only on platforms nusb supports
+# (Linux, macOS, Windows). Android/Termux uses target_os="android" and is excluded.
+[target.'cfg(any(target_os = "linux", target_os = "macos", target_os = "windows"))'.dependencies]
+nusb = { version = "0.2", default-features = false, optional = true }
+
 # probe-rs for STM32/Nucleo memory read (Phase B)
 probe-rs = { version = "0.30", optional = true }