Expose embedded camera timecode via ExtendScript

Expose embedded camera timecode (Start Timecode from Interpret Footage) via ExtendScript on FootageItem or FootageSource. Currently FootageItem.displayStartTime returns NaN for proprietary formats (R3D, ARRIRAW, etc.) even though AE reads and displays the correct timecode in the UI and in the Footage viewer. A reliable FootageItem.sourceStartTimecode property (in seconds or frames) would allow scripts to generate accurate EDL-style reports for legal/rights clearance workflows — which is a common production need.