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/state_registry.h

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+/*
+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 <functional>
+#include <optional>
+#include <string>
+#include <unordered_map>
+#include <vector>
+
+#include "hardware/protocol/protocol_defs.h"
+#include "vehicle/Train.h"
+
+// shared TrainState registry
+// one description of the simulator state exposed to the outside world, meant to be used by every
+// external interface (hardware protocol v2, zmq, python) instead of each of them keeping its own list
+
+namespace hardware
+{
+
+// values captured once per simulation frame, so that all consumers see a consistent set
+struct state_snapshot
+{
+	// false when there's no occupied vehicle; state symbols then report UNAVAILABLE
+	bool available = false;
+	// high voltage circuit is meaningful for this vehicle
+	bool electric = false;
+	// vehicle is equipped with pantographs
+	bool pantographs = false;
+	TTrain::state_t train {};
+	// last active cab, 0: front, 1: rear. moving to the engine room doesn't change it
+	std::uint8_t cab_indicator = 0;
+	// changes whenever the controlled vehicle changes, used to notify devices about metadata changes
+	std::uint32_t context_id = 0;
+	double time_month_of_era = 0.0;
+	double time_minute_of_month = 0.0;
+	double time_millisecond_of_day = 0.0;
+};
+
+struct state_value
+{
+	value_type type = value_type::float32;
+	value_quality quality = value_quality::unavailable;
+	double numeric = 0.0;
+	std::string text;
+};
+
+struct state_descriptor
+{
+	// registry name without namespace prefix, i.e. "velocity" rather than "state.velocity"
+	std::string name;
+	value_type type = value_type::float32;
+	std::string unit;
+	std::optional<double> minimum;
+	std::optional<double> maximum;
+	std::function<state_value( state_snapshot const & )> getter;
+};
+
+class state_registry
+{
+
+public:
+// methods
+	static state_registry const &instance();
+
+	std::vector<state_descriptor> const &entries() const { return m_entries; }
+	state_descriptor const *find( std::string const &Name ) const;
+
+	// fills the snapshot from the currently occupied train; simulation thread only
+	static void capture( state_snapshot &Snapshot );
+
+private:
+// methods
+	state_registry();
+	void add( std::string Name, value_type const Type, std::string Unit, std::optional<double> const Minimum, std::optional<double> const Maximum, std::function<state_value( state_snapshot const & )> Getter );
+
+// members
+	std::vector<state_descriptor> m_entries;
+	std::unordered_map<std::string, std::size_t> m_index;
+};
+
+} // namespace hardware