docs(specs): record the Linux native-video compositing spike

Adds the spike write-up and re-opens finding 2 of
playback-backend-unification.md, which concluded that video cannot unify
on a native engine because the webview owns the surface.

That finding's general form has since been falsified on Android, where
native video composites behind a transparent WebView and ships on by
default. The evidence behind it was also entirely about foreign-window
embedding -- mpv's render API, drawing into a GL context we own inside
Tauri's own GTK tree, was never tested. The spike tests that one claim
and comes back green on Linux for both X11 and Wayland, bar the Tauri
default_vbox() half of G1.

Findings 3-6 are deliberately left standing. Finding 3 in particular --
mpv has no adaptive bitrate -- is an independent disqualifier that a
green compositing result does not clear, and the spike says so rather
than reading as a green light.

The next-free-id line moves to UR-079 / IR-033 / DR-219: this branch
allocated UR-077 and UR-078 for the updater and diagnostics work, and
DR-215 through DR-218 with them, after that line was last written.

Authored in a parallel session working in the same checkout; committed
here so it travels with the rest of the branch.

docs/specs/README.md

@@ -27,8 +27,8 @@ know how something *works*, read
 | Design authority | No code of its own — it records a decision later specs act on. |
 
 **Next free requirement ids** (always re-check
-[requirements.md](../requirements.md) before allocating): **UR-077**,
-**IR-033**, **DR-215**. Three specs below suggested ids that have since been
+[requirements.md](../requirements.md) before allocating): **UR-079**,
+**IR-033**, **DR-219**. Three specs below suggested ids that have since been
 taken by other work; each carries a ⚠️ note at the top.
 
 ## Partially implemented
@@ -47,6 +47,7 @@ taken by other work; each carries a ⚠️ note at the top.
 | [build-provenance.md](build-provenance.md) | `build.rs` is still bare. ⚠️ suggested id DR-093 is taken. |
 | [player-facade-enforcement.md](player-facade-enforcement.md) | ~60 `commands.player*` sites still outside the facade; no lint rule. ⚠️ suggested id DR-095 is taken. |
 | [windows-native-audio-backend.md](windows-native-audio-backend.md) | Blocked on the libmpv2 swap. ⚠️ suggested id IR-030 is taken. |
+| [linux-native-video-spike.md](linux-native-video-spike.md) | **Spike run 2026-08-21: compositing works on Linux, X11 and Wayland.** G1-G6 green bar the Tauri `default_vbox()` half of G1. Needs an implementation spec that answers adaptive bitrate. |
 
 ## Design authority