docs: say that HrtRateFit::toSeconds returns producer-epoch, not wall, seconds

The fit keeps the slope and discards the intercept, so the result counts from
the producer's boot. Tasks 4 and 7 compose it with ClockOffset::map, which is
correct, but the bare name invites passing it straight to a plot axis.

Also unwrapped the stalled-clock assertion from behind `if (fit.ready())` —
that branch never runs, so the test confirmed nothing.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Client/udpscope/TimeBase.h

@@ -63,6 +63,15 @@ public:
     void add(uint64_t hrt, double wallSec);
     bool ready() const { return n_ >= kMinSamples && rate_ > 0.0; }
     double ticksPerSecond() const { return rate_; }
+    /**
+     * @brief Converts a tick count to seconds on the PRODUCER's own epoch.
+     *
+     * The fit recovers the slope only and discards the intercept, so this is
+     * `hrt / ticksPerSecond()` — not a wall-clock time. A producer's hrt counts
+     * from its own boot, not from the Unix epoch. Pass the result to
+     * ClockOffset::map() to land it on the wall clock; latching that arbitrary
+     * epoch difference is precisely what ClockOffset is for.
+     */
     double toSeconds(uint64_t hrt) const;
     void reset();