hardware protocol v2: frame layer and shared symbol registry

Adds the transport-independent core of the MaSzyna Hardware Protocol v2:
COBS framing with a CRC32C protected 24 byte header, bounds checked
payload readers and writers, and a resynchronising stream decoder.

The shared TrainState registry describes simulator state in engineering
units with per-value quality, the command registry generates the command
namespace from Commands_descriptions, and the symbol registry merges both
into the public, name-addressed hardware api.

hardware/registry/command_registry.h

@@ -0,0 +1,67 @@
+/*
+This Source Code Form is subject to the
+terms of the Mozilla Public License, v.
+2.0. If a copy of the MPL was not
+distributed with this file, You can
+obtain one at
+http://mozilla.org/MPL/2.0/.
+*/
+
+#pragma once
+
+#include <optional>
+#include <string>
+#include <unordered_map>
+#include <vector>
+
+#include "hardware/protocol/protocol_defs.h"
+#include "input/command.h"
+
+// command namespace of the hardware api
+// the list is generated from simulation::Commands_descriptions, with hardware metadata added on top,
+// so that external devices never depend on the numeric values of the user_command enumeration
+
+namespace hardware
+{
+
+// optional conversion applied to the value received from the device before the command is posted
+enum class command_transform : std::uint8_t
+{
+	none = 0,
+	// 0...1 fraction of the master controller range of the occupied vehicle
+	mastercontroller_percent = 1
+};
+
+struct command_descriptor
+{
+	// registry name without namespace prefix, i.e. "trainbrakeset" rather than "command.trainbrakeset"
+	std::string name;
+	user_command command = user_command::none;
+	// true for controls which may be driven with CONTROL_SET, false for event-only commands
+	bool continuous = false;
+	value_type parameter_type = value_type::float32;
+	std::optional<double> minimum;
+	std::optional<double> maximum;
+	command_transform transform = command_transform::none;
+};
+
+class command_registry
+{
+
+public:
+// methods
+	static command_registry const &instance();
+
+	std::vector<command_descriptor> const &entries() const { return m_entries; }
+	command_descriptor const *find( std::string const &Name ) const;
+
+private:
+// methods
+	command_registry();
+
+// members
+	std::vector<command_descriptor> m_entries;
+	std::unordered_map<std::string, std::size_t> m_index;
+};
+
+} // namespace hardware