Improve preview responsiveness and audio playback stability

[jonoomph]

Improve preview startup, seeking, and audio continuity while making playback shutdown and reader changes safer.

Changes

• Responsive caching: Cancel obsolete work promptly, preserve useful cached frames, and resume caching for the latest request.
• Adaptive pre-roll: Adjust buffering to frame generation time and cache capacity. Recover missing frames without discarding the cached frames after them.
• Clean audio after seeking: Discard stale audio after seeks or transport changes; safely retain and release audio sources across stop and restart.
• Safer reader changes: Drain work before replacing readers and wake stopped render workers to avoid hangs and unsafe lifetime changes.
• Test and licensing cleanup: Add missing copyright headers, remove the new Markdown writeup, and address static-analysis findings in tests and benchmark helpers.

Validation

• Added regression tests and reproducible pre-roll benchmarks.
• Passed 45 focused native tests after cleanup. Reader-handoff Memcheck, the library/bindings rebuild, and Python import also passed.
• REUSE passes: All 418 library files have copyright and license information.
• Still outstanding: Cross-platform and physical audio-device reconnect testing. Validation so far is local Linux.

Build notes and companion PR

• Test together with openshot-qt #6172: Faster edits, Properties dock positioning, and optimized trim/razor previews.

[jonoomph]

Improve preview responsiveness and audio playback stability