fix(inject/linux/ds4): fold the Linux DS4 backend onto the shared proto codec (3.3.0)

The Linux DualShock 4 backend missed the G2-era shared-mapping work and
drifted from dualshock4_proto three ways, leaving two user-visible gaps
on the DS4 kind (Windows, written later against the proto, is correct):

- its serialize_state duplicated the proto's byte-for-byte EXCEPT byte 7:
  raw st.buttons[2] instead of buttons2_with_click(), so a rich-plane pad
  click never reached the report;
- its inline apply_rich never set touch_click and dropped the Steam LEFT
  pad entirely (surface 1 skipped), where the shared
  dualsense_proto::DsState::apply_rich splits the one touchpad left/right;
- handle() didn't preserve touch_click across button-only frames.

Net effect: Deck client -> Linux host on the DS4 kind = pad clicks and
the left pad dead.

Delete the local serialize_state/parse_ds4_output/Ds4Feedback/pack_touch
and touch-dim consts in favor of dualshock4_proto (dropping the proto's
keep-in-sync FIXME), route rich events through the shared
DsState::apply_rich, and preserve touch_click in the frame merge exactly
like the other three DS-family managers. The proto's serialize_offsets
test gains a touch_click case pinning byte 7 bit 1.

Verified on .21: cargo clippy -p punktfunk-host --all-targets -D warnings
clean; full suite 277 pass / 0 fail.

Pre-step 3.3.0 of the G12 skeleton extraction (gamepad-review-cleanup.md
§3a.2) — the behavior fix lands before the mechanical dedup.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

crates/punktfunk-host/src/inject.rs

@@ -485,8 +485,8 @@ pub mod dualsense_windows;
 #[cfg(target_os = "linux")]
 #[path = "inject/linux/dualshock4.rs"]
 pub mod dualshock4;
-/// Transport-independent DualShock 4 HID codec used by the Windows UMDF-driver backend
-/// ([`dualshock4_windows`]). (The Linux backend still carries its own copy — see the module FIXME.)
+/// Transport-independent DualShock 4 HID codec, shared by the Linux UHID backend ([`dualshock4`])
+/// and the Windows UMDF-driver backend ([`dualshock4_windows`]).
 #[cfg(any(target_os = "linux", target_os = "windows"))]
 #[path = "inject/proto/dualshock4_proto.rs"]
 pub mod dualshock4_proto;