`gamescope_ours_and` answered "did WE spawn this gamescope?" by reading `PUNKTFUNK_GAMESCOPE_NODE`.
Phase 2.3 deleted the code that published that key — routing.rs's own doc says "Nothing is written
back to the two knobs" — but this consumer was never migrated, so the read now returns "not
attaching" for every attach.
Both consumers then answer for a session this host has no flags on. On a plain box with a foreign
gamescope already running, `pick_gamescope_mode` resolves Attach at its fifth rung while the env key
stays unset, and the probe half only inspects the resolved BINARY, which is our patched build:
* `gamescope_composites_cursor()` returns true, so the host attaches no XFixes reader and blends
nothing — while the stock gamescope actually running was never given
`--pipewire-composite-cursor`, so the stream carries no pointer at all.
* `gamescope_hdr_available()` returns true, so the Welcome fixes `bit_depth` at 10 and the session
negotiates BT.2020/PQ over an 8-bit SDR composite. The Welcome cannot take that back.
The same two failures hit the `capture_monitor` mirror route on any Bazzite or SteamOS box, where
the running Game Mode gamescope is by definition not one this host spawned.
The question is now asked of the resolved route rather than the environment, via a pure
`session_is_a_foreign_gamescope` that runs — and is tested — on every platform. The residual gap is
named in the doc rather than papered over: `create_managed_session`'s create-time degrade to a
foreign attach is still invisible to a ladder re-run.
Also in this commit:
* Two unguarded session-env reads now take `ENV_LOCK` (`detect()`'s `XDG_CURRENT_DESKTOP` fallback
and `effective_topology()`'s legacy pins). `apply_session_env` `set_var`s those same keys from
another thread, which is the glibc setenv/getenv race this crate's own lib.rs documents as UB.
* `mirror.rs`'s `names_ours_conclusively` was a `matches!` whose omitted default was the UNSAFE
direction — a new backend would silently get its own virtual displays mirrored. Now exhaustive, so
adding a `Compositor` is a compile error at the one site where the answer is a safety decision.
* `MirrorDisplay` overrides `poolable_now() -> false`; its `create` always reports `External`, so
the trait's `true` default was a pre-create claim contradicting the post-create fact. The trait
doc now says plainly that the default is a default and not a fact.
* The crate front-door doc listed 3 of 7 backends and quoted line counts half the size of the
current crate; `routing.rs`'s summary was attached to the wrong item and described a published env
channel that no longer exists; `available()` is no longer documented as cheap when it forks
`gamescope --version` and does an unbudgeted Wayland roundtrip per call.
enricobuehler
a1ff0dde0c