feat: add Xbox One HID report layout and mapping

Phase 4 foundation, independent of the presentation mechanism:

- hid/hid_report.hpp: standard Xbox One report layout (64-byte input:
  buttons/guide/triggers/sticks; 64-byte output: motors), the HID
  report descriptor, and GIP state to report mapping.
- make_input_report(): remaps GIP button bits to the standard Xbox
  bitmask, scales 10-bit triggers to 8-bit, sticks pass through.
- parse_output_report(): extracts motor intensities from output
  reports for the rumble relay; malformed reports stop the motors.
- test_hid: unit tests for the mapping, scaling, and descriptor.

Report packing verified with a descriptor walker: input and output
are exactly 64 bytes each. Next step is presentation via DriverKit,
which needs a Developer ID with the DriverKit entitlement.

Co-Authored-By: qwen3.8-27b@q3_k_xl: implemented HID report layer

PLAN.md

@@ -49,6 +49,10 @@ and exposes connected controllers as HID gamepads.
 - C API + Swift app: async session (fast probe; firmware + radio on a worker
   thread), state display (idle/starting/ready/error), controller list with
   live input monitor, battery level, and a rumble test button.
+- Phase 4 (HID) started: standard Xbox One report layout, HID report
+  descriptor, and GIP-to-report mapping implemented in `src/hid/` and unit
+  tested. Remaining: present the reports as a virtual device via a DriverKit
+  extension (needs a Developer ID with the DriverKit entitlement).
 
 The dongle LED is driven at chip level for pairing and client lifecycle.
 Phase 3 is complete. Next: Phase 4 (virtual HID gamepad) and Phase 5 (app
@@ -153,7 +157,7 @@ Extract the protocol logic from Linux kernel code into standalone C.
 - `ieee80211_*` → custom 802.11 frame builders
 - `cfg80211_*` → nothing (no regulatory domain reporting needed)
 
-### Phase 4: Virtual HID Gamepad (new)
+### Phase 4: Virtual HID Gamepad (in progress)
 
 **src/hid/**
 
@@ -164,10 +168,16 @@ Extract the protocol logic from Linux kernel code into standalone C.
   - **IOHIDSystem user-space**: Create virtual HID device entirely in
     user-space. May not work for all games.
   - **Gamepad wrapper**: Lower-level, translate input events to HID reports.
-- Map Xbox controller buttons/sticks/triggers to standard Xbox 360/One HID
-  report descriptor
-- Handle force feedback (rumble): send back to dongle via GIP
+- Map Xbox controller buttons/sticks/triggers to standard Xbox One HID
+  report descriptor: done. `hid/hid_report.hpp` defines the 64-byte
+  input/output report layout and the HID report descriptor; GIP state maps
+  to input reports via `make_input_report()` (unit tested).
+- Handle force feedback (rumble): send back to dongle via GIP. TX is done
+  (Phase 3); output-report parsing for the relay is implemented in
+  `parse_output_report()`.
 - Battery status reporting
+- Remaining: present the reports as a virtual device (DriverKit extension).
+  Needs a Developer ID with the DriverKit entitlement to build and load.
 
 ### Phase 5: macOS App (new)