fix: correct PCAN timestamps (uptime-based backends)

[minoseigenheer]

Problem

PCAN/PeakCAN hardware provides timestamps relative to device power-on (uptime), not the Unix epoch. The existing code subtracted timeBasis (a Unix epoch value in µs) from this small uptime value, causing unsigned integer underflow and producing large negative-looking timestamps (e.g. -1777429996593330 µs).

A secondary problem existed even after detecting the uptime clock: using a QDateTime wall-clock anchor to convert the PCAN uptime to session-relative time introduced a systematic ~3 ms bias (macOS monotonic vs. wall clock drift ~14 ppm). This made every Rx timestamp appear slightly after its corresponding Tx timestamp in override-mode delta calculations, breaking the expected request/response order.

A third problem: clicking Clear Frames while frames are arriving restarts mElapsedTimer, but frames already queued by the hardware thread carry timestamps from the old timer epoch. Those frames are drained ~20 ms later by the refresh tick and appear with incorrect (large) timestamps relative to subsequent frames.

Fix

Commit 1 — correct PCAN timestamps for uptime-based backends

• Detect uptime-based hw timestamps by checking hwTimestamp < timeBasis.
• For those backends, use CANConManager::getElapsedUs() directly instead of the PCAN hw clock. This is the identical QElapsedTimer monotonic source used for Tx timestamps, so Tx and Rx are always on the same clock with zero drift or bias.
• Add CANConManager::getElapsedUs() helper (exposes mElapsedTimer.nsecsElapsed() / 1000).
• Remove first-frame anchor members (mFirstHwTimestamp, mFirstElapsedUs) that are no longer needed.

Commit 2 — prevent incorrect timestamps after Clear Frames

• resetTimeBasis() now flushes every connection's frame queue immediately after restarting mElapsedTimer, so pre-reset frames are discarded before the drain timer fires.

Files changed

• connections/serialbusconnection.cpp — uptime detection + getElapsedUs() for Rx
• connections/serialbusconnection.h — removed obsolete anchor members
• connections/canconmanager.cpp — added getElapsedUs(), queue flush in resetTimeBasis()
• connections/canconmanager.h — added getElapsedUs() declaration

Testing

Verified with a PCAN-USB on macOS:

• Timestamps are now positive and start near 0 µs on session start.
• Rx timestamps align with Tx timestamps (same monotonic source, no fixed bias).
• Clicking Clear Frames while live traffic is flowing produces correct timestamps immediately after the clear.