Fix live UART data loss and time-axis glitches

[kaklik]

The live serial reader rebuilt the full time/sum/spectral history from scratch on every incoming record, making per-record work grow with session length; once that fell behind the incoming baud rate, the OS serial buffer could overflow and drop bytes. It also used readline() with a timeout, which silently truncates a line that arrives slower than the timeout, corrupting or dropping that record. Parsing failures were swallowed with bare except: pass, so none of this was visible.

• Replace the growing-list-to-array rebuild with amortized O(1) append buffers (_GrowableArray/_GrowableMatrix) so per-record cost stays constant over a session.
• Read raw bytes and split on '\n' ourselves instead of relying on readline(timeout=...), so a slow/bursty record is only parsed once it has fully arrived.
• Log dropped/malformed records instead of discarding them silently.
• Drop the $STOP timestamp fallback that substituted the run index for time (compressing the x-axis); fall back to host wall-clock time only when the device itself reports no timestamp (no RTC).