feat(host/steam): harden the gadget feature contract — fixes the evdev churn

The virtual Deck's gamepad evdev was churning (destroyed + recreated) because
Steam kept re-probing: GetControllerInfo reads HID feature reports, and the gadget
served zeros for them. Captured the real contract off a physical Deck
(packaging/linux/steam-deck-gadget/get_deck_attrs.c, hidraw HIDIOCGFEATURE — usbmon
truncates to 32B) and implemented it in steam_gadget.rs::feature_reply:

- 0x83 GET_ATTRIBUTES_VALUES: [83, 2d, 9×(attr-id, u32-LE)] — product id 0x1205, a
  per-instance unit serial (0x0a/0x04, so a gadget never collides with a real Deck
  or another gadget), and the capability attrs (0x09=0x2e, 0x0b=0x0fa0, rest 0).
- 0xAE GET_STRING_ATTRIBUTE: [ae, len, attr, ascii] — serial (attr 1) / board
  serial (attr 0).
- other commands (0x87 settings): echo the last write.

Validated on the Deck: 1 connect / 0 disconnect / 1 gamepad evdev (was constant
churn), Steam activates the gadget cleanly (no GetControllerInfo failed, no zombie)
and emits its X-Box 360 pad. usbmon on the gadget's bus confirms our state reports
(pressed button at byte 8) are delivered on the interrupt-IN and consumed by
hid-steam — so with M1/M2's byte-8→BTN_SOUTH decode the input chain is proven
end-to-end. Remaining: a foreground-game confirmation of Steam Input's XInput
mapping, then default the gadget on for SteamOS.

Workspace clippy/fmt/test green. Not pushed.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packaging/linux/steam-deck-gadget/get_deck_attrs.c

@@ -0,0 +1,39 @@
+// Query a physical Steam Deck's feature reports (SET command then GET response) over hidraw to get
+// the FULL blobs (usbmon truncates to 32 bytes). Steam feature reports are unnumbered 64-byte; the
+// hidraw buffer prefixes a report-id byte (0).
+#include <linux/hidraw.h>
+#include <sys/ioctl.h>
+#include <fcntl.h>
+#include <stdio.h>
+#include <string.h>
+#include <unistd.h>
+static void dump(const char *name, unsigned char *b, int n) {
+    printf("%s rc=%d:", name, n);
+    for (int i = 0; i < (n > 0 ? n : 0); i++) printf(" %02x", b[i]);
+    printf("\n");
+}
+int main(int argc, char **argv) {
+    if (argc < 2) { printf("usage: %s /dev/hidrawN\n", argv[0]); return 1; }
+    int fd = open(argv[1], O_RDWR);
+    if (fd < 0) { perror("open"); return 1; }
+    // SET [reportid=0, cmd, len, attr, ...] then GET the response.
+    unsigned char queries[][4] = {
+        {0x83, 0x00, 0x00, 0x00}, // GET_ATTRIBUTES_VALUES
+        {0xae, 0x16, 0x01, 0x00}, // GET_STRING_ATTRIBUTE, serial (attr 1)
+        {0xae, 0x16, 0x00, 0x00}, // GET_STRING_ATTRIBUTE, attr 0
+        {0xae, 0x16, 0x02, 0x00}, // attr 2 (board serial?)
+    };
+    for (int q = 0; q < 4; q++) {
+        unsigned char set[65] = {0};
+        set[0] = 0; // report id 0
+        memcpy(set + 1, queries[q], 4);
+        int sr = ioctl(fd, HIDIOCSFEATURE(65), set);
+        usleep(3000);
+        unsigned char get[65] = {0};
+        get[0] = 0;
+        int gr = ioctl(fd, HIDIOCGFEATURE(65), get);
+        printf("=== query cmd=%02x attr=%02x  (SET rc=%d) ===\n", queries[q][0], queries[q][2], sr);
+        dump("  GET", get, gr);
+    }
+    return 0;
+}