fix: a loaded recording is filtered into its buffer, not truncated

The loader pushed one slot per file row while the plan decimated by
eight, so a 10 M-row file overran the ring and only its last 4.19 s
survived -- a 20 s recording that read as a 4 s one. Each group of
`time_stride` rows is now averaged into one slot at the group's mean
stamp, as the packet path already does, and the load line reports
rows and decimation so a filtered file cannot be mistaken for a
truncated one.

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

docs/claude/probe-readings.md

@@ -979,7 +979,8 @@ configuration that carries settings, is the defect this exists for — a saved h
 [sources] chooser ui: page='sources' title='Sources' icon=yes page='waveforms' title='Waveforms' icon=yes visible='sources' switcher_wired=1 buttons=[prefs=1 new_host=1 open=1 cancel=1]
 [sources] waveforms: 1 recent /tmp/rt.npy(ok) page=waveforms
 [sources] picked waveform /tmp/rt.npy
-[udpscope] waveform loaded: /tmp/rt.npy signals=4 samples=20001 span=2.000000s rate=10000 time=[-2.000000..0.000000]
+[udpscope] waveform loaded: /tmp/rt.npy signals=4 samples=20001 rows=20001 decim=1 span=2.000000s rate=10000 time=[-2.000000..0.000000]
+[udpscope] waveform loaded: /tmp/long.npy signals=4 samples=31250 rows=1000000 decim=32 span=19.999980s rate=50000 time=[-4.000000..15.999980]
 ```
 
 | field | value it must read | the defect a wrong reading names |
@@ -987,7 +988,9 @@ configuration that carries settings, is the defect this exists for — a saved h
 | `chooser ui:` | both pages with a `title` and an `icon=yes`, `switcher_wired=1`, and all four buttons `1` | the chooser's `.ui` losing an id: the code names the missing one and refuses the window, and this line is the frame that *did* build -- a page with `icon=NO` is one the switcher cannot draw |
 | `waveforms:` | one entry per remembered export, with `(ok)` or `(missing)`, and `page=waveforms` once the page is shown | the Waveforms page not built, or a stale path offered as if it were openable |
 | `picked waveform` | the selected path | a pick that carries no file: `--open` is what the mode reads |
-| `signals=`, `samples=` | the export's `cols=` and `rows=` | a reader mis-slicing its records, or the columns bound to the wrong channels |
+| `signals=`, `rows=` | the export's `cols=` and `rows=` | a reader mis-slicing its records, or the columns bound to the wrong channels |
+| `samples=` × `decim=` | `rows`, to within the last partial group | the loader not filtering the file into the plan it built: every row pushed into a ring that cannot hold them leaves the file as its own tail (`stored=` far above `published=` in the channel dump, and a 20 s recording on screen as its last 4 s) |
+| `decim=` | 1 whenever the recording fits the buffer at its own rate; a power of two above it | a decimation that moved without the span moving — the second line above is the reading to compare against: `samples` fell 32-fold and `span=` did **not** |
 | `span=`, `rate=`, `time=[…]` | the exported window and rate, `[-window, 0]` | the plan not covering the whole file (the tail arrives as the whole picture) or the display epoch not pinned to the file's last stamp |
 | `frames=0 packets=0` beside `channels=N` | 0 — no acquisition | a socket opened for a file, which would also make the starvation notice live |