Grilling: lock the v3 media-capture spec

Part of #78

Blocked by: [Research: MPRIS event fingerprint — what sidra actually emits](https://github.com/adityanandanx/heimdall/issues/79), [Grilling: one media session — boundaries, wall-clock, acceptance contract](https://github.com/adityanandanx/heimdall/issues/81), [Prototype: sidra no-compression SessionTracker](https://github.com/adityanandanx/heimdall/issues/82), [Grilling: extension URL contract for watch-sessions](https://github.com/adityanandanx/heimdall/issues/83)

## Question

Lock the **v3 media-capture spec**: the session model + acceptance contract from the session grilling, the sidra no-compression tracker from the prototype, the URL source from the extension contract — one coherent spec replacing the MPRIS/CDP clause of the v2 spec (#20).

The map closes with this ticket resolved; implementation is a separate follow-up effort.