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.

Read #20 and the linked blocking issues #79, #81, #82, and #83 first to gather the session model, acceptance contract, tracker, and URL-source requirements. Locate the existing media-capture spec document in this repository and update it so the v3 spec is a single coherent replacement for the v2 MPRIS/CDP clause. No tests are specified; done means the revised spec is clearly written, agreed on as the locked version, and notes implementation is deferred to a follow-up issue.

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