The Linux native plane has been 8-bit for a reason that stopped being true:
`capturer_supports_hdr()` returned a flat `false` because Mutter's virtual
monitors are SDR-only upstream. That is still right for Mutter, KWin and
wlroots — and wrong for gamescope, whose node can now offer 10-bit BT.2020 PQ
(packaging/gamescope). Open the three gates that held it off, together:
* the capture-side gate becomes SOURCE-AWARE (`capturer_supports_hdr_for`):
Windows keeps its platform answer, gamescope asks whether the resolved binary
offers the formats, everything else stays false. It must be truthful rather
than optimistic — the Welcome is irrevocable, and PQ frames on an 8-bit
encoder are a deliberate hard error — so every term is a static fact resolved
before anything is spawned, including "do we spawn gamescope at all" (an
attach inherits someone else's flags and can promise nothing);
* `want_hdr` reaches the capturer: `open_virtual_output` grew the parameter it
never had, and the host arm stopped dropping `want.hdr` on the floor;
* the spawn gains `--hdr-enabled --hdr-debug-force-support` on all three
sub-modes (bare args, the `GAMESCOPE_BIN` wrapper, the SteamOS PATH shim),
from one shared `hdr_args` — the force flag is what makes it work headless,
and forgetting it looks exactly like a negotiation failure.
Two things that would have gone wrong quietly:
* the keep-alive reuse key gains `hdr`. gamescope cannot turn HDR on live, so a
kept SDR display handed to an HDR session would give the game no HDR surfaces
while the stream negotiated PQ over an SDR composite — wrong, and not
obviously broken. Same for the managed session-plus/SteamOS reuse check.
* the HDR-negotiation-failure latch becomes PER SOURCE. It was one
process-wide flag, so a monitor that left HDR mode would have disabled
gamescope HDR until the host restarted, and vice versa — two facts with
nothing in common but the word HDR.
GameStream follows the same shape: `host_hdr_capable` gains the gamescope arm,
and the RTSP gate's live BT.2100 monitor probe is scoped to the portal source —
a headless box has no monitor to be in HDR mode, so running it there would
hard-refuse every gamescope HDR session.
Off by default for one release (`PUNKTFUNK_GAMESCOPE_HDR=1`), and
`punktfunk-host hdr-probe` now answers for both Linux HDR sources.
enricobuehler
689297c86e