Measure dropped samples from stream timestamps instead of hiding them

Overflows were counted but the samples the driver dropped were simply
missing, so a 5 s capture with 300 overflows still read as a clean 5.000 s
file. Each read's hardware timestamp is now compared with where the
previous read ended; gaps are sized exactly, zero-filled by default so the
file timeline stays true, and reported as completed_with_gaps with a
warning, samples_dropped and a gap list. This also catches drops the
plugin never flags as overflows (libusb transfer timeouts).

Timestamps are differenced before rounding: rounding each absolute
timestamp to a sample index flips by +/-1 near half-sample boundaries
and produced phantom 1-sample gaps.

- stream buffers 128 x 64K samples (~280 ms at 30 Msps, was ~4 ms)
- reject out-of-range gain, frequency, rate and bandwidth instead of
  letting the plugin clamp silently; default bandwidth clamps to range
- spectrum returns a max-hold decimated summary (256 points) plus
  noise_floor_db; full arrays were 139 KB at fft_size 4096
- capture_iq background=True with stream_status polling, so
  stop_capture is reachable; device-changing tools refuse while it runs
- OSError returned in the error shape; status drops "unknown" fields
- README: gap accounting, keep captures off tmpfs; captures/ ignored

.gitignore

@@ -6,3 +6,4 @@ __pycache__/
 dist/
 *.cf32
 *.cs16
+captures/