v0.32.0 gave per-app prep do/undo steps PF_STREAM_WIDTH/_HEIGHT/_REFRESH/_HDR
(hooks::prep_mode_env, both serving planes), but the docs' only example was a
one-liner. Add the entry+script pair that shows the point: the prep entry stops
naming a device and reads the negotiated mode instead - an FPS cap at the
client's own refresh, an HDR arm taken only when the session negotiated it, and
the raster for whatever wants pixels.
Written so the traps are visible rather than explained: undo sees the values its
do saw, HDR is 1/0 (the marker file's spelling) not PF_EVENT_*'s true/false, the
app identity is PF_APP_ID or PF_APP_TITLE by plane, and set -u makes an older
host fail the step loudly instead of capping at 'fps_limit='.
Verified: the fenced script extracted from the doc and run under sh - do writes
the cap and takes the HDR arm, undo restores and reverses it, an SDR session
skips HDR, an empty identity falls through to the literal, and an unset
environment exits non-zero.
enricobuehler