G8 of the gyro program, SDL-client half.
The `Welcome` has always carried the backend the host actually RESOLVED, which
is not necessarily the one the client asked for — Auto lands on Xbox 360 for
anything not Sony/Valve/Xbox, and a Switch Pro on a Windows host folds to X360
too. No client read the field. So a player with an 8BitDo, or a Switch Pro on
Windows, got a controller whose gyro did nothing, with nothing anywhere saying
why: the client shipped ~250 Hz of Motion datagrams and the host parsed and
discarded every one.
`GamepadPref::has_motion()` answers whether a backend has a motion plane at all.
The SDL client checks it on the first gyro sample: it logs one line naming the
resolved backend and pointing at the fix (pick a DualSense-class controller
type), then stops sending. Once per slot, not per sample — this path runs at the
pad's sensor rate.
`Auto` deliberately answers true. It means "unknown" — an old host that omitted
the echo, which may well have resolved a DualSense — and suppressing motion on
unknown would silently break working gyro, a worse failure than sending
datagrams nobody reads. The predicate is an exhaustive match so a new backend
has to state its answer rather than inherit one, and a table test pins both
halves: a false negative kills working motion, a false positive keeps the void
open, and both are silent.
Owed: the plan wants this surfaced as a one-line UI hint, not just a log line.
Apple already stores `resolvedGamepad` and Android needs the plumb; neither is
done here, and both want their own gate.
Gate (Linux CI image): fmt, build, clippy --all-targets -D warnings, and the
test suites — green, with the new capability test observed running.
enricobuehler
77797a9e20