ripping with genuine GW hardware, a typical track is read in
to gw as
```
Flux: 72.00 MHz
Total: 232202 samples, 594.17ms
Revolution 0: 93.06ms
Revolution 1: 166.87ms
Revolution 2: 166.87ms
Revolution 3: 166.87ms
```
our firmware came in as
```
Flux: 24.00 MHz
Total: 120485 samples, 652.35ms
Revolution 0: 0.00ms
Revolution 1: 162.34ms
Revolution 2: 55.11ms
Revolution 3: 162.17ms
Revolution 4: 55.28ms
Revolution 5: 162.12ms
```
Notice how there's an empty revolution 0 and a partial revolution
2 & 4.
It appears that
* if capture_ms is 0, nothing is captured after the index
(rather than 50ms)
* no index is emitted at the start (though this is because
the first revolution is almost always a partial revolution)
Now after this change, a track on rp2040 looks like
```
Flux: 24.00 MHz
Total: 96498 samples, 502.46ms
Revolution 0: 0.00ms
Revolution 1: 167.49ms
Revolution 2: 167.49ms
Revolution 3: 167.49ms
```
which appears to be closer to what gw expects; it also fixes how
g64conv used to require `r1` to select a particular revolution.
This may have been because it defaulted to one of the 'runt'
revolutions, which also made it misestimate the rotational speed
of the drive.
.. after awhile it would run out of program space, because
it was not freeing the right program.
also add support for terminating a read after a given number
of ms, or 50ms after the second index pulse.
This nearly works with fluxengine (slightly older revision):
```
$ ./fluxengine write ibm1440 -i bloop.img --usb.serial=...
Measuring rotational speed... 200ms
Writing to: drive 0
0.0: Write: 199 ms in 76988 bytes
Verify: 391 ms in 150010 bytes
0.1: Write: 199 ms in 92225 bytes
Verify: 326 ms in 150006 bytes
1.0: Write: 199 ms in 79272 bytes
Verify: 380 ms in 150002 bytes
```
but at some point, the fw and fluxengine stop successfully
talking:
```
Measuring rotational speed... 200ms
Writing to: drive 0
0.0: Write: 199 ms in 76988 bytes
Verify: 391 ms in 150010 bytes
0.1: Write: 199 ms in 92225 bytes
Verify: 326 ms in 150006 bytes
1.0: Write: 199 ms in 79272 bytes
Verify: 380 ms in 150002 bytes
```
