feat(clients/input): system buttons route around local overlays

Pressing guide/Steam/QAM collided with the client device's own shell: iOS 26
opens its Game Overlay for the Home press (no app opt-out until iOS 27 makes
it a user setting), and a Gaming-Mode client opened BOTH Steam overlays for
one press — the local one covering the stream.

Two cross-client tier-P settings, zero wire changes:

- system_buttons (auto|forward|local): raw guide+misc1 passthrough. Auto
  forwards everywhere EXCEPT under gamescope, where SteamOS reacts to the
  same physical press no matter what.
- guide_gesture (auto|on|off): hold Select ALONE ~350ms sends the HOST's
  guide, down until release — held on, that's the host's long-press, which
  opens a Gaming-Mode host's QAM for regular pads. A Select tap is delivered
  on release with its up TAP_PRESS (50ms) behind, because per-transition
  sends fold into seq'd GamepadState snapshots and a back-to-back pair can
  coalesce into no press at all. A Select inside a combo (the escape chord)
  passes through untouched. Auto arms it only where the raw press can't
  reach the host cleanly: gamescope, iOS/iPadOS, tvOS.

The same SelectGesture rules live in pf-client-core (pure state machine +
unit tests), the Apple client (mask-diff adaptation in GamepadCapture), and
Android's GamepadRouter. Settings rows on every surface (GTK, WinUI,
console UI, Decky, Apple x2, Android x2) with profile plumbing throughout.

punktfunk-session grows a control socket
($XDG_RUNTIME_DIR[/app/$FLATPAK_ID]/punktfunk-session-ctl.sock — the one
runtime path a flatpak and the host see identically): 'guide'/'qam' verbs
inject synthetic taps. The Decky panel gains a Host menus section (visible
while the client runs) whose buttons press the host's Steam/QAM and close
the local menu so the host's shows through.

iOS 27's GCControllerHomeButtonSettingsManager deep-link is a TODO (the
class needs the Xcode 27 SDK to compile). Docs: input, client-settings,
steam-deck. Design: punktfunk-planning design/system-buttons-routing.md.

Gates: docker clippy --all-targets --locked -D warnings + tests
(pf-client-core 88 incl. 6 new gesture tests, pf-console-ui 47),
cargo fmt --all --check, swift build (macOS), gradle kit+app compile,
decky tsc --noEmit + py_compile. clients/windows not compiled (no box).

docs-site/content/docs/client-settings.md

@@ -193,6 +193,21 @@ which forwards *every* connected controller, each as its own player, on Linux, W
 console home. Pinning one restricts the session to that controller alone — single-player. The Android
 app has no such picker.
 
+**Steam / guide button** (*Guide button* on Apple and Android) — *default: Automatic*, on every
+client. Where the guide (Xbox/PS/Steam) and quick-access presses go while streaming: **Send to
+host** forwards them raw, **This device** keeps them local. Automatic forwards everywhere except
+Gaming Mode, where SteamOS opens its own menus for those buttons no matter what — forwarding raw
+there opens *both* menus at once, the local one covering the stream. The full story, including how
+to reach the host's menus when the raw press stays local, is on the
+[Input page](/docs/input#the-guide-button-xbox--ps--steam-and-quick-access).
+
+**Hold Select for guide** — *default: Automatic*, on every client. The gesture that presses the
+host's guide button from any controller: hold Select (Back/View) on its own for about a third of a
+second, and keep holding for the host's long-press (a Gaming-Mode host's Quick Access Menu, on a
+regular pad). Automatic arms it only where the raw guide press can't reach the host cleanly —
+Gaming Mode, iPhone/iPad, Apple TV — because the gesture has a cost: a Select *tap* arrives a beat
+late, and a game that expects a *held* Select would trigger it. Set **On** or **Off** to overrule.
+
 **Capture system shortcuts** — *default: on.* Offered by the Linux and Windows apps, the console home
 and Decky; Windows spells the row out as *Capture system shortcuts (Alt+Tab, Win, …)*. On a Deck it
 matters only for a keyboard you attached yourself, for the reason the paragraph below gives: Gaming