Keep vanetza-idf in obu-firmware, so a plain clone builds the firmware

obu-firmware builds against the vanetza-idf C-ITS library, which until now
came from the colleague's microbu-esp32c5 tree beside the repository and was
not tracked here, so a clone of this repository could not build the firmware
it ships. The library alone is now part of obu-firmware, as
obu-firmware/external/vanetza-idf: their external/vanetza-idf at commit
cf4b99f, unchanged (9775 files; see its PROVENANCE.md). CMake takes it from
there by default; -DVANETZA_IDF_DIR still points the build elsewhere.

The rest of the colleague's tree (their own VAM firmware, PKI tooling,
station-link Python tools, the V2X2MAP bridge) stays out of this repository
and gitignored; nothing is pushed to their repository. NOTES.md, docs/06,
TODO.md and the pcap verifier's usage line point at the new location.

obu-firmware/external/vanetza-idf/vanetza/common/unit_interval.cpp

@@ -0,0 +1,89 @@
+#include <vanetza/common/unit_interval.hpp>
+#include <algorithm>
+#include <cmath>
+#include <limits>
+
+namespace vanetza
+{
+
+UnitInterval& UnitInterval::operator+=(const UnitInterval& other)
+{
+    m_value += other.m_value;
+    return clamp();
+}
+
+UnitInterval& UnitInterval::operator-=(const UnitInterval& other)
+{
+    m_value -= other.m_value;
+    return clamp();
+}
+
+UnitInterval& UnitInterval::operator*=(const UnitInterval& other)
+{
+    m_value *= other.m_value;
+    // all unit interval multiplications remain within range
+    return *this;
+}
+
+UnitInterval& UnitInterval::operator/=(const UnitInterval& other)
+{
+    m_value /= other.m_value;
+    // only upper limit has to be enforced
+    m_value = std::min(m_value, 1.0);
+    return *this;
+}
+
+UnitInterval& UnitInterval::operator+=(double value)
+{
+    m_value += value;
+    return clamp();
+}
+
+UnitInterval& UnitInterval::operator-=(double value)
+{
+    m_value -= value;
+    return clamp();
+}
+
+UnitInterval& UnitInterval::operator*=(double value)
+{
+    m_value *= value;
+    return clamp();
+}
+
+UnitInterval& UnitInterval::operator/=(double value)
+{
+    m_value /= value;
+    return clamp();
+}
+
+bool UnitInterval::operator<(const UnitInterval& other) const
+{
+    return m_value < other.m_value;
+}
+
+bool UnitInterval::operator==(const UnitInterval& other) const
+{
+    // epsilon should be fine for values in [0.0, 1.0]
+    return std::abs(m_value - other.m_value) < std::numeric_limits<double>::epsilon();
+}
+
+UnitInterval& UnitInterval::clamp()
+{
+    m_value = clamp(m_value);
+    return *this;
+}
+
+UnitInterval UnitInterval::complement() const
+{
+    UnitInterval complement;
+    complement.m_value = 1.0 - m_value;
+    return complement;
+}
+
+UnitInterval mean(UnitInterval lhs, UnitInterval rhs)
+{
+    return UnitInterval { 0.5 * (lhs.value() + rhs.value()) };
+}
+
+} // namespace vanetza