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/inject/proto/steam_proto.rs

@@ -542,12 +542,16 @@ pub fn deck_unit_id(index: u8) -> u32 {
     0x5046_0000 | index as u32
 }
 
-/// A Steam-accepted alphanumeric unit serial (a real Deck's is e.g. `"FVZZ4200469B"`; Steam rejects
-/// a too-short/oddly-formatted one as "Invalid or missing unit serial number" and substitutes its
-/// own — benign, but we present a clean 12-char one). Derived from [`deck_unit_id`] so the `0xAE`
-/// serial reply and the `0x83` unit-id attrs stay consistent.
+/// A Steam-accepted alphanumeric unit serial (a real Deck's is e.g. `"FVZZ4200469B"`). Steam
+/// validates the serial's FORMAT before accepting it: a `"PF"`-leading serial is REJECTED
+/// ("Invalid or missing unit serial number …") and Steam then substitutes a hash AND mangles the
+/// displayed controller name (observed as "Steam Deck Controllerggg" on Windows). An `'F'`-leading
+/// serial passes, so we keep the PunktFunk marker one slot in (`"FVPF"`) — still distinct from a
+/// real Deck's `"FVZZ"` for the self-detection below while satisfying Steam's format check.
+/// Derived from [`deck_unit_id`] so the `0xAE` serial reply and the `0x83` unit-id attrs stay
+/// consistent. (The Windows UMDF driver mirrors this exact format — see pf-dualsense lib.rs.)
 pub fn deck_serial(index: u8) -> String {
-    format!("PFDK{:08X}", deck_unit_id(index))
+    format!("FVPF{:08X}", deck_unit_id(index))
 }
 
 /// The neutral 64-byte Deck input report (header only, all controls released) — the report the
@@ -914,7 +918,7 @@ mod tests {
     fn deck_feature_reply_contract() {
         let serial = deck_serial(0);
         let unit_id = deck_unit_id(0);
-        assert_eq!(serial, "PFDK50460000"); // 12-char alphanumeric, derived from the unit id
+        assert_eq!(serial, "FVPF50460000"); // 12-char alphanumeric, derived from the unit id
         assert_eq!(serial.len(), 12);
 
         // 0x83 GET_ATTRIBUTES_VALUES: header + (0x0a, unit_id) at the 3rd attribute slot.

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
         }