Complete Phase 1: add clock utility, unit test, build targets, and docs

include/screencast/utils/clock.h

@@ -0,0 +1,19 @@
+#pragma once
+
+#include <cstdint>
+
+namespace sc {
+
+// RTP clock rate for video is 90 kHz. Convert a monotonic nanosecond timestamp
+// into a 32-bit RTP timestamp. Overflows wrap around intentionally.
+constexpr uint32_t rtp_timestamp_from_ns ( uint64_t nanoseconds ) noexcept {
+    // (nanoseconds * 90_000) / 1_000_000_000
+    // Simplify: 90_000 / 1_000_000_000 = 9 / 100_000
+    return static_cast<uint32_t> ( ( nanoseconds * 9ULL ) / 100'000ULL );
+}
+
+static_assert ( rtp_timestamp_from_ns ( 0 ) == 0 );
+static_assert ( rtp_timestamp_from_ns ( 1'000'000'000ULL ) == 90'000 );
+static_assert ( rtp_timestamp_from_ns ( 500'000'000ULL ) == 45'000 );
+
+}  // namespace sc