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

tests/meson.build

@@ -1 +1,5 @@
-# Empty stub: tests will be added as modules land.
+test_clock = executable('test_clock',
+    'utils/test_clock.cpp',
+    include_directories : sc_core_inc)
+
+test('clock utils', test_clock)

tests/utils/test_clock.cpp

@@ -0,0 +1,20 @@
+#include "screencast/utils/clock.h"
+
+#include <cassert>
+#include <cstdint>
+
+int main () {
+    assert ( sc::rtp_timestamp_from_ns ( 0 ) == 0u );
+    assert ( sc::rtp_timestamp_from_ns ( 1'000'000'000ULL ) == 90'000u );
+    assert ( sc::rtp_timestamp_from_ns ( 500'000'000ULL ) == 45'000u );
+    assert ( sc::rtp_timestamp_from_ns ( 16'666'667ULL ) == 1'500u );
+
+    // A multiple of the wrap period should wrap back to zero.
+    // Period in ns = ceil(2^32 * 100_000 / 9) because the helper uses
+    // integer floor division (ns * 9 / 100_000).
+    constexpr uint64_t period_ns = ( ( 1ULL << 32 ) * 100'000ULL + 9ULL - 1ULL ) / 9ULL;
+    static_assert ( sc::rtp_timestamp_from_ns ( period_ns ) == 0u );
+    static_assert ( sc::rtp_timestamp_from_ns ( period_ns + 1'000'000'000ULL ) == 90'000u );
+
+    return 0;
+}