fix: store the stream clock as f64, so a stamp's precision does not decay with uptime

The time ring was f32 like the values, holding each stamp relative to an epoch
fixed at the first sample ever seen -- so the number stored grew for as long as
the app ran while the interval it had to resolve stayed one sample period. At
1 MS/s the f32 ULP reaches the sample period after about sixteen seconds, so two
to four consecutive samples rounded onto one stamp after half a minute and eight
after a minute. On screen that is a plateau the LOD envelope draws as a
zero-width segment -- a vertical spike where the waveform should be -- and in an
export it was three distinct diffs at 2, 2.5 and 3 ULP instead of one clock.

A value's precision is a property of the signal; a timestamp's is a property of
how long the scope has been running. Only the clock widens: the values are the
memory budget. That makes the two slices different sizes per slot, so
plan_partition() weighs its growth loop in bytes rather than in slots (and
bytes_for() is what bounds the doubling); live_window_locked() loses the f32
round-trip of its bounds, which existed only to keep a sample sitting exactly on
one; and EventSnapshot::stamps is f64 because a pause view is not rebased and so
carries full stream time.

Verified against the fast mock: np.diff over two million exported rows now holds
exactly one step (1e-06, 1999999 times), no duplicate or backwards stamps, and
the 200 MB ring still spans 8.39 s. Every documented probe assertion unchanged.

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

src/channel.cpp

@@ -24,7 +24,7 @@ const plot::data::MinMaxPyramid& null_pyramid() {
 // --- ChannelBuffer ----------------------------------------------------------
 
 ChannelBuffer::ChannelBuffer(std::shared_ptr<const plot::data::RingBuffer<f32>> values,
-                             std::shared_ptr<const plot::data::RingBuffer<f32>> times, u64 stride,
+                             std::shared_ptr<const plot::data::RingBuffer<f64>> times, u64 stride,
                              f64 epoch, f64 nominal_dt)
     : values_(std::move(values)), times_(std::move(times)), stride_(stride < 1 ? 1 : stride),
       epoch_(epoch), dt_(nominal_dt) {}
@@ -64,11 +64,11 @@ void ChannelBuffer::acquire() noexcept {
 
 void ChannelBuffer::invalidate() noexcept {
   published_ = 0;
-  window_x0_ = 0.0f;
+  window_x0_ = 0.0;
   pyramid_.clear();
 }
 
-f32 ChannelBuffer::time_of(u64 absolute_index) const noexcept {
+f64 ChannelBuffer::time_of(u64 absolute_index) const noexcept {
   // Channel sample `j` is produced on the tick that also pushes time slot
   // `j*stride + stride - 1` (see `Acquisition::on_data`): the producer stamps the
   // time ring every `time_stride` ticks and the channel every `stride` of those,
@@ -102,10 +102,10 @@ f32 ChannelBuffer::time_of(u64 absolute_index) const noexcept {
   // whole trace's geometry collapsing.
   const u64 base = times_->base();
   const u64 produced = times_->produced();
-  if (produced == 0) return 0.0f;
+  if (produced == 0) return 0.0;
   u64 slot = index < base ? base : index;
   if (slot >= produced) slot = produced - 1;
-  f32 out = 0.0f;
+  f64 out = 0.0;
   times_->copy_from(slot, 1, &out);
   return out;
 }
@@ -121,7 +121,7 @@ f64 ChannelBuffer::dx() const {
 
 f64 ChannelBuffer::x0() const {
   if (pyramid_.size() == 0) return epoch_;
-  return epoch_ + f64(window_x0_);
+  return epoch_ + window_x0_;
 }
 
 f64 ChannelBuffer::x_at(u64 i) const {
@@ -131,7 +131,7 @@ f64 ChannelBuffer::x_at(u64 i) const {
   // ring-span too new made it refuse the whole visible range -- which left the renderer
   // gathering the entire ring at a coarse level and drawing it off the axis.
   if (i == 0) return x0();
-  return epoch_ + f64(time_of(absolute(i)));
+  return epoch_ + time_of(absolute(i));
 }
 
 bool ChannelBuffer::neighbours(f64 x, u64& i0, u64& i1) const {
@@ -171,7 +171,7 @@ u64 ChannelBuffer::snap_index(f64 x) const {
 plot::Range ChannelBuffer::x_bounds() const {
   const u64 n = pyramid_.size();
   if (n == 0) return plot::Range{epoch_, epoch_};
-  return plot::Range{x0(), epoch_ + f64(time_of(absolute(n - 1)))};
+  return plot::Range{x0(), epoch_ + time_of(absolute(n - 1))};
 }
 
 plot::Range ChannelBuffer::y_bounds() const {