WARN-1: pumpRunning now reset to false when the pump loop exits via
{done} or error, so future startPump() isn't a silent no-op.
WARN-3: readExact clears rxBuffer on timeout to avoid stale bytes
leaking into the next non-retried STK500 command.
readExact and drainStray now consume the FIFO buffer fed by the pump
instead of calling reader.read() directly. No orphaned reader.read()
promise can swallow optiboot INSYNC/OK replies anymore. This is the
core fix for "get_sync failed after N attempts".
Single pump loop becomes the only reader.read() caller, feeding
rxBuffer. Serial monitor + readExact/drain will consume from the
buffer (next tasks). Build passes.
a2nr