[WIP] Add time stretch and pitch shift data model

[Copilot]

Thanks for assigning this issue to me. I'm starting to work on it and will keep this PR's description up to date as I form a plan and make progress.

Original prompt

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This section details on the original issue you should resolve

<issue_title>feat: Time stretch / pitch shift data model</issue_title>
<issue_description>Add to Clip: timeStretchRate (1.0=original), pitchShift (semitones). Store actions: setClipTimeStretch, setClipPitchShift. 2 unit tests.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes feat: Time stretch / pitch shift data model #148

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[ChuxiJ]

Closing — superseded by #178 (feat: time stretch + pitch shift data model), which is the active PR for issue #148.