Research: extension URL payload — the CDP contract replacement

Part of #78

## Question

What does our browser extension already emit about the active page, and can `media_url`/`media_source` for watch-sessions be sourced from it instead of CDP (which is being removed)?

Inspect `extensions/` + the native messenger (`com.heimdall.messenger`, `src/heimdall/native_messenger`): payload shape (tab url / video id / title / position?), frequency, gating (capture paused?), and how to correlate with MPRIS sessions (chromium.playerName vs tab). Compare with the fields today's CDP path (`_enrich_chromium_media` + `cdp_resolve`) provides, so the replacement fills the same holes (title-only sessions gain url/video id).

Deliverable: `docs/research/extension-url-payload.md` (branch `research/extension-url-payload`) + a gap list: which CDP fields have no extension equivalent today. Blocks the extension-contract grilling.