fix(gamepad/windows): Steam-accepted Deck unit serial un-mangles the controller name

Steam validates the Deck unit serial's format before accepting it. Our
"PFDK..." serial was REJECTED ("Invalid or missing unit serial number"), so
Steam substituted a hash identity and mangled the displayed name to
"Steam Deck Controllerggg" on every host tested. An 'F'-leading serial passes,
so switch to "FVPF..." — keeps the PunktFunk marker one slot in, still distinct
from a real Deck's "FVZZ..." for the Linux self-detection in
physical_steam_controller_present(). The name now shows a clean "Steam Deck
Controller" with a serial-derived handle (verified on .173).

Also fix the UMDF driver's 0xAE GET_STRING_ATTRIBUTE handler to echo the
requested attribute id faithfully instead of collapsing board-serial (0x00)
requests to unit-serial (0x01). Steam still logs a benign "Deck Controller PCB
Serial# invalid" for the board serial — it validates that against a
Valve-internal format for ANY value, including an empty one (verified) — but
that line does not mangle the name, change the handle, or block promotion.

Applied to both transports: host inject/proto/steam_proto.rs::deck_serial
(Linux gadget/usbip) and the pf-dualsense UMDF driver (Windows), which mirror
each other's serial format.

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

crates/punktfunk-host/src/punktfunk1.rs

@@ -2865,7 +2865,7 @@ fn degrade_if_no_uhid(chosen: GamepadPref) -> GamepadPref {
 /// device (vhci resolves through `vhci_hcd`, NOT `/devices/virtual/`), so a just-ended session's
 /// pad still detaching — or a concurrent session's live one — read as "physical" and degraded
 /// every back-to-back Deck session to DualSense (observed live on Bazzite 2026-07-04). Ours are
-/// recognizable by the `PFDK…` serial ([`steam_proto::deck_serial`]) in `HID_UNIQ`, with the
+/// recognizable by the `FVPF…` serial ([`steam_proto::deck_serial`]) in `HID_UNIQ`, with the
 /// vhci path as belt and braces.
 #[cfg(target_os = "linux")]
 fn physical_steam_controller_present() -> bool {
@@ -2877,7 +2877,7 @@ fn physical_steam_controller_present() -> bool {
             return false;
         }
         if std::fs::read_to_string(e.path().join("uevent"))
-            .is_ok_and(|u| u.lines().any(|l| l.starts_with("HID_UNIQ=PFDK")))
+            .is_ok_and(|u| u.lines().any(|l| l.starts_with("HID_UNIQ=FVPF")))
         {
             return false; // one of our own virtual Decks
         }