fix(data): draw the partial edge buckets at LOD levels, correct the pyramid's memory and precondition docs

Findings from the review of the sliding-window change:

- A level >= 1 gather emitted only complete, wholly-retained buckets, so the
  geometry stopped at the last bucket boundary inside the range: the newest
  samples of a live window -- up to `span - 1` of them, about a pixel at the
  level level_for() picks -- were not drawn, on the right edge, which is the
  part of a scrolling trace that gets watched, and the oldest retained samples
  fell outside the geometry too (the level-1 gather of a sliding window started
  51 samples in). The two partial edge buckets are now gathered as raw samples
  through copy_raw() and bracketed around the envelope -- the same exactness
  split reduce/sum/sumsq already use -- so the emitted geometry spans the
  requested range exactly. The pyramid still stores no partial bucket, so append
  stays amortised O(1); at 1 MS/s the demo stages 511 extra sub-pixel segments
  and still draws at 1.05 ms/frame, and the screenshot's trace now reaches one
  column further at each end.
- `drop_before`'s doc claimed a no-op "for an empty pyramid": false, and
  backwards -- the restart branch is exactly what an empty pyramid hits, which is
  what StreamSeries::acquire() depends on when the ring saturated before the
  first acquire. The precondition is `absolute_sample <= window_base()`, and the
  restart is documented as what it is.
- The pyramid's memory figure was 3x optimistic: an entry is 24 bytes (MinMax
  plus the exact sum/sumsq doubles), so level 0 costs 0.375n bytes and the whole
  pyramid ~0.75n -- about 19% of 4-byte raw samples, not the 6% a min/max-only
  index would cost. Corrected in the header, spec 4.2 and 4.5, with the trade
  stated (the doubles buy O(log n) sum/mean/RMS/integral from whole buckets),
  and the plan's Task 3 sketch no longer describes per-level prefix sums, a
  level() span or an unaligned-end widening that does not exist.

Tests: `gather_at_a_lod_level_reaches_both_ends_of_the_window` pins that the
first and last emitted points are the oldest and newest retained samples -- with
spike values no bucket envelope can produce -- at level 1, at the level a dense
view picks, and for a static series whose end is unaligned. It fails against the
previous commit (first point at the first whole bucket), and so does the sliding
gather mapping check, which now accounts for the raw edge pairs.

src/data/series.cpp

@@ -117,25 +117,29 @@ f64 Series::sumsq(u64 begin, u64 end) const {
   return total;
 }
 
+void Series::gather_raw(u64 begin, u64 end, std::vector<f32>& out_xy) const {
+  const f64 x_0 = x0();
+  const f64 step = dx();
+  f32 raw[kGatherChunk];
+  for (u64 i = begin; i < end;) {
+    const u64 take = std::min<u64>(kGatherChunk, end - i);
+    copy_raw(i, i + take, raw);
+    for (u64 k = 0; k < take; ++k) {
+      out_xy.push_back(f32(x_0 + f64(i + k) * step));
+      out_xy.push_back(raw[k]);
+    }
+    i += take;
+  }
+}
+
 void Series::gather_impl(u32 level, u64 begin, u64 end, std::vector<f32>& out_xy) const {
   const u64 n = size();
   const u64 b = std::min(begin, n);
   const u64 e = std::min(std::max(end, b), n);
   if (b >= e) return;
 
-  const f64 x_0 = x0();
-  const f64 step = dx();
   if (level == 0) {
-    f32 raw[kGatherChunk];
-    for (u64 i = b; i < e;) {
-      const u64 take = std::min<u64>(kGatherChunk, e - i);
-      copy_raw(i, i + take, raw);
-      for (u64 k = 0; k < take; ++k) {
-        out_xy.push_back(f32(x_0 + f64(i + k) * step));
-        out_xy.push_back(raw[k]);
-      }
-      i += take;
-    }
+    gather_raw(b, e, out_xy);
     return;
   }
 
@@ -146,23 +150,39 @@ void Series::gather_impl(u32 level, u64 begin, u64 end, std::vector<f32>& out_xy
   // are both complete and wholly inside the window: a bucket that is only partly
   // retained aggregates samples the window no longer has, and a partial trailing
   // bucket is not stored at all.
+  //
+  // Those two excluded buckets are still part of the range, and one of them holds
+  // the newest samples -- for a live trace, the part being watched. They are drawn
+  // from the raw samples instead: at most one bucket at each end, about a pixel
+  // each at the level level_for() picks, so the envelope path stays O(buckets)
+  // and the geometry still spans the requested range exactly.
+  const f64 x_0 = x0();
+  const f64 step = dx();
   const u64 span = u64{kBucket} << level;
   const u64 origin = pyramid().window_base();
   const u64 lbase = pyramid().level_base(level);
   const u64 lend = lbase + pyramid().level_size(level);
-  if (lbase >= lend) return;
-  const u64 a_end = origin + e;
   const u64 first = std::max((origin + b) / span, lbase);
-  const u64 last = std::min((a_end - 1) / span + 1, lend);
+  const u64 last = std::min((origin + e - 1) / span + 1, lend);
+  if (first >= last) {
+    // No complete bucket of this level is inside the range -- a window squeezed
+    // into one bucket -- so the raw samples are the only exact geometry there is.
+    gather_raw(b, e, out_xy);
+    return;
+  }
+  const u64 lead = first * span - origin;   // window index the first emitted bucket starts at
+  if (b < lead) gather_raw(b, std::min(lead, e), out_xy);
   for (u64 bi = first; bi < last; ++bi) {
     const MinMax m = pyramid().bucket(level, bi);
     const u64 first_sample = bi * span - origin;   // >= 0: stored buckets start inside the window
-    const u64 last_sample = std::min((bi + 1) * span, a_end) - 1 - origin;
+    const u64 last_sample = std::min((bi + 1) * span, origin + e) - 1 - origin;
     out_xy.push_back(f32(x_0 + f64(first_sample) * step));
     out_xy.push_back(m.min);
     out_xy.push_back(f32(x_0 + f64(last_sample) * step));
     out_xy.push_back(m.max);
   }
+  const u64 trail = last * span - origin;   // one past the last emitted bucket's span
+  if (trail < e) gather_raw(trail, e, out_xy);
 }
 
 }  // namespace plot::data