Fix copy/paste transition causes bad audio

[bmatherly]

As reported here:
https://forum.shotcut.org/t/bug-with-dissolve-on-copied-track/51766/

[Copilot]

Pull request overview

This PR updates multi-clip paste operations in TimelineDock to better preserve transitions when copying/pasting a track/tractor, addressing a reported issue where pasted transitions could cause incorrect audio.

Changes:

• Detect transition “mix” clips in pasted MLT playlists and skip inserting them as normal clips.
• Recreate skipped transitions via AddTransitionByTrimOutCommand after inserting adjacent clips.
• Adjust inserted clip in/out points and insertion positions to account for skipped transition entries.

Comments suppressed due to low confidence (3)

src/docks/timelinedock.cpp:4538

• A transition is added whenever pendingTransitionDuration > 0, even if the transition tractor's right-side clip could not be resolved (pendingTransitionRightIn < 0 / pendingTransitionRightClip invalid) or does not match the current clip. In those cases this can add a transition between the wrong pair of clips. Gate the transition creation on successfully matching the current clip to the transition's right clip, but still reset the pending-transition state either way.

                            if (pendingTransitionDuration > 0) {
                                const int rightClipIndex = clipIndexAtPosition(trackIndex,
                                                                               overwritePosition);
                                const int leftClipIndex = rightClipIndex - 1;
                                if (leftClipIndex >= 0 && rightClipIndex >= 0

src/docks/timelinedock.cpp:4532

• This code now overwrites clips at overwritePosition (= position + info.start - skippedTransitionDuration), but the group-restore pass later in this function still looks up overwritten clips using position + clipInfo.start. When transitions are skipped (skippedTransitionDuration > 0), those lookups can resolve to the wrong clip (or blank), causing copied group membership to be restored incorrectly for pastes that include transitions.

                            const int overwritePosition = position + info.start
                                                          - skippedTransitionDuration;
                            MAIN.undoStack()->push(new Timeline::OverwriteCommand(m_model,
                                                                                  trackIndex,
                                                                                  overwritePosition,
                                                                                  MLT.XML(&clip),
                                                                                  false));

src/docks/timelinedock.cpp:4303

• A transition is added whenever pendingTransitionDuration > 0, even if the transition tractor's right-side clip could not be resolved (pendingTransitionRightIn < 0 / pendingTransitionRightClip invalid) or does not match the current clip. In those cases this can add a transition between the wrong pair of clips. Gate the transition creation on successfully matching the current clip to the transition's right clip, but still reset the pending-transition state either way.

                            if (pendingTransitionDuration > 0) {
                                const int rightClipIndex = clipIndexAtPosition(trackIndex,
                                                                               insertPosition);
                                const int leftClipIndex = rightClipIndex - 1;
                                if (leftClipIndex >= 0 && rightClipIndex >= 0

[ddennedy]

sheez, these methods are huge. Is there an opportunity to make a reusable method in the added code?