feat(wake): add controller-free USB serial wake beacon

Add a standalone Pico2W image reusing the configured nonconnectable wake
burst without Bluepad32, controller-host profiles, Classic/LE-central
roles, connection pools, pairing or writable TLV storage. Explicitly
disable and verify Classic inquiry/page scan before declaring readiness.
Keep the original controller firmware and captured wake identity unchanged.

Expose bounded SPWB1 CDC commands with exactly-once request IDs, explicit
failures and radio progress independent of serial backpressure. Add the
cross-platform switch-pico-wake script with strict preflight, bounded I/O,
no automatic rebroadcast, status-only mode and preserved uncertain-outcome
metadata. Standard USB serial requires no WinUSB/Zadig binding.

Add isolated --wake-only build/publication selection and document setup,
backup and physical-BOOTSEL recovery. Validate776 tests, focused framing
and lifecycle cases, ELF isolation and actual CDC operation on only the
new board. One burst completed in2.095s; malformed/status/reconnect input
and same-ID replay caused no additional broadcast. Original Pico untouched.
Private firmware images, console configuration and backups are excluded.

tools/switch2_wake_beacon/beacon_protocol.h

@@ -0,0 +1,62 @@
+#pragma once
+
+#include <stddef.h>
+#include <stdint.h>
+
+#include "input/switch2_wake.h"
+
+namespace wake_beacon {
+
+struct RadioStatus {
+    bool ready = false;
+    bool failed = false;
+    bool initialized = false;
+    Switch2WakeDiagnostics wake{};
+};
+
+// Owned entirely by Core 0. Parsing only queues a request; the radio owner calls
+// dispatch_pending()/dispatched() separately, outside USB callbacks and IRQs.
+class Protocol {
+public:
+    static constexpr size_t kMaxLineBytes = 64;  // Excludes LF, includes optional CR.
+    static constexpr size_t kResponseBytes = 512;
+
+    void connected(bool connected);
+    bool can_receive() const;
+    bool receive(uint8_t byte);
+    const char* output_data() const;
+    size_t output_size() const;
+    void consume_output(size_t count);
+
+    void observe(const RadioStatus& radio);
+    bool dispatch_pending() const;
+    void dispatched(bool accepted);
+
+private:
+    enum class State : uint8_t {
+        Idle, Queued, Broadcasting, Complete, Unconfigured, Failed,
+    };
+
+    bool active() const;
+    bool busy() const;
+    const char* state_name() const;
+    void command();
+    void respond(const char* error);
+
+    RadioStatus radio_{};
+    State state_ = State::Idle;
+    uint32_t request_id_ = 0;
+    uint32_t accepted_requests_ = 0;
+    uint32_t local_failures_ = 0;
+    uint32_t start_completed_ = 0;
+    uint32_t start_failures_ = 0;
+    bool connected_ = false;
+    bool invalid_line_ = false;
+    char line_[kMaxLineBytes]{};
+    size_t line_size_ = 0;
+    char response_[kResponseBytes]{};
+    size_t response_size_ = 0;
+    size_t response_offset_ = 0;
+};
+
+}  // namespace wake_beacon