feat: the export writes an MDF 3.30 file, so a window loads in ScryLab

A third format beside `.npy` and `.csv`, written by hand in `src/export.cpp` --
MF4 is HDF5 and would mean a library, MF3 is a handful of fixed blocks. The
layout is the asammdf 8.8.27 reader's own field list, byte-diffed against a
known-good 3.30 file it wrote; the shape is the one ScryLab's data-source
contract asks for: one channel group, record-interleaved, the master as the x
axis, and the unit in the channel's conversion block.

So in MDF the master is `abs_seconds`, named `time` -- the clock that is the
same in every display mode -- and the plotted axis goes out as an ordinary
channel, `time_relative`. `ExportTable::Column` gained the CONFIG's unit, which
only this writer spells out; the `.npy`/`.csv` bytes are unchanged, and
`write_export()` now picks the writer for both the dialog and the probe.

docs/claude/verification.md

@@ -15,7 +15,7 @@ xvfb-run -a env UDPSCOPE_EXIT_AFTER_MS=8000 UDPSCOPE_ENABLE_ALL_MS=1000 \
 
 Two baselines every run is read against. A clean build emits **four** warnings and no more: two
 `-Wmissing-field-initializers` on `MathNode::source` and `-Woverlength-strings` on **both**
-generated GResource `.c` files — the help pages' (**514660** bytes) and the icons', whose blob is
+generated GResource `.c` files — the help pages' (**515700** bytes) and the icons', whose blob is
 **12352** against C99's 4095-byte string-literal limit with the seven action icons and the two app icons
 that remain (a note about a generated file, which GCC itself supports) -- re-measured when
 the tab work changed the set (that icon left, the two tab glyphs arrived, and this file's
@@ -194,8 +194,8 @@ precedent, not an exception.
   nothing. Its four lines are in
   [probe-readings.md](probe-readings.md).
 - `UDPSCOPE_EXPORT_MS` (+ optional `UDPSCOPE_EXPORT_PATH`, default
-  `/tmp/udpscope-export`) — collect the export table and write it both ways, skipping
-  only the file chooser.
+  `/tmp/udpscope-export`) — collect the export table and write it all three ways (`.npy`,
+  `.csv`, `.mf3`), skipping only the file chooser.
 
 - `UDPSCOPE_PROBE_CONDITION_MS` — arms a qualified trigger against the mock and reports the latched
   `t0`, the measured pulse or band width, and sample values for the window condition. Schedules its