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.

src/export.hpp

@@ -21,6 +21,11 @@ namespace udpscope {
 struct ExportTable {
   struct Column {
     std::string name;         ///< the legend name, sanitised for a column label
+    /// The signal's unit exactly as the CONFIG carried it, the empty string when it
+    /// carried none. The `.npy`/`.csv` writers do not spell it out (their readers get the
+    /// names and the numbers); MDF does, because the unit *is* part of what a channel is
+    /// there, and it is the only thing ScryLab reads a signal's unit from.
+    std::string unit;
     std::vector<f32> values;  ///< `rows()` long, the ring's own `f32`
   };
 
@@ -71,4 +76,27 @@ bool write_npy(const std::string& path, const ExportTable& table, std::string& e
 /// enough digits to round-trip each column's own type and no more.
 bool write_csv(const std::string& path, const ExportTable& table, std::string& error);
 
+/// Writes the same table as **MDF 3.30** (`.mf3`/`.mdf`), byte for byte by hand -- there is no
+/// writer library here and MF4 (HDF5) is the one format this app will not link.
+///
+/// One channel group, all channels record-interleaved, master first, which is the shape the
+/// [ScryLab data sources](https://docs.scrylab.de/docs/data-sources/) contract asks for: *every
+/// data channel becomes a signal* (the CNBLOCK's `short_name`), *the master channel of each
+/// group is the x axis* (our master is `abs_seconds`, named `time`), and *the unit is taken from
+/// the channel metadata* (MDF 3 keeps it in the channel's conversion block, the CCBLOCK). A
+/// second occurrence of a name is dropped by that reader, so the labels are de-duplicated and cut
+/// to the 31 bytes a CNBLOCK's name field holds.
+///
+/// The master is the **absolute** clock and the axis the plot showed is an ordinary channel,
+/// `time_relative`: the relative column is `0` at a different sample in every mode, so a master
+/// off it would put two exports from different modes on two timelines, while `abs_seconds` is the
+/// number the stream itself stamped the samples with. Both carry the unit `s`.
+bool write_mdf(const std::string& path, const ExportTable& table, std::string& error);
+
+/// Writes `table` with the writer **`path`'s extension** names: `.csv` -> `write_csv`,
+/// `.mf3`/`.mdf` -> `write_mdf`, anything else -> `write_npy` (the default, as the save dialog's
+/// default filter is). The export dialog and the self-check's export probe both come through
+/// here, so the two cannot offer different formats.
+bool write_export(const std::string& path, const ExportTable& table, std::string& error);
+
 }  // namespace udpscope