Add AMD FSR 4 upscaling through the libraries of AMD's FSR SDK 2.4, as an Upscaling choice beside FSR 3.1.

runtime/vk_ffx4.cpp loads amd_fidelityfx_loader_vk.dll from beside the executable and asks it which upscaler
versions the device can run. Where a 4.x version is listed (Radeon RX 6000, RX 7000 and RX 9000 cards with AMD's
driver 26.10.1 or newer) the menu's Upscaling row gains AMD FSR 4 Native AA, Quality, Balanced, Performance and
Ultra Performance, and fsr4 = 1 runs it in FSR 3.1's place with the same inputs. Its contexts are made with that
version asked for by its id and the provider is asked afterwards which version it runs, so no other upscaler can
be shown under the name. When a context or a dispatch fails, FSR 3.1 makes the frame, and after three failures
FSR 4 is off for the run. Each upscaler starts its history anew on the frame after a change from one to the other.
The setting is off by default and the Linux build compiles stubs.

On an RX 7900 XT (driver 26.10.1, 3440x1440, HDR) the library runs FSR 4.1.2: a context takes 54 ms to make and
217 MiB of video memory. Standing in Hyrule Field: Quality 47.8 fps where FSR 3.1 gives 51.5, Native AA 41.4.
Screenshots of 601 consecutive frames standing, 801 during a camera sweep, 701 each in Kakariko and the
graveyard, and 1,301 across changes between FSR 3.1 and FSR 4, their modes and off show no flicker that FSR 3.1
does not have, and it runs together with FSR 3.1 frame generation.

The SDK's API headers are in runtime/third_party/ffx_sdk, unmodified, with the licence. tools/make_kit.py and
tools/make_launcher.py take AMD's two signed libraries with --ffx4, and tools/make_game.py copies them beside the
game when the kit has both.

Co-Authored-By: Claude Fable 5.2 <noreply@anthropic.com>

docs/README-BuildKit.md

@@ -112,7 +112,8 @@ cannot give back what the game needs after its start: about 10 GB in Kokiri Fore
 
 AMD FSR 3.1 loads AMD's `amd_fidelityfx_vk.dll`, which only the Windows build can use: on Linux the menu offers neither
 FSR 3.1 upscaling nor FSR 3.1 frame generation, and `taa = fsr3` or `framegen = fsr3` in `nx2navi.ini` leave the
-game's own scaling and swapchain as they are (the log says so).
+game's own scaling and swapchain as they are (the log says so). AMD FSR 4 is in the Windows build alone for the same
+reason, and `fsr4 = 1` does nothing on Linux.
 
 NVIDIA DLSS (RTX cards) needs the driver's own `libnvidia-ngx.so.1` on the library path (the proprietary driver's
 utilities package, `nvidia-utils` on Arch) and NVIDIA's DLSS library `libnvidia-ngx-dlss.so.<version>` beside
@@ -151,7 +152,8 @@ beside the game.
 - **Screen** in the menu has Texture filtering (anisotropic, up to 16x, at once), Texture detail (Reduced and Low
   leave out the largest versions of the big surface textures so that the game fits the video memory of 6 and 8 GB
   cards; Automatic picks by the card), Upscaling (DLSS on NVIDIA RTX cards,
-  FSR 3.1 on any card where `amd_fidelityfx_vk.dll` loads), Frame generation (NVIDIA DLSS on RTX 40 and 50 cards, with
+  FSR 3.1 on any card where `amd_fidelityfx_vk.dll` loads, FSR 4 on the Radeon cards AMD's FSR 4 libraries list it
+  for), Frame generation (NVIDIA DLSS on RTX 40 and 50 cards, with
   a DLSS multiplier of 2x, or 3x and 4x on RTX 50 cards; FSR 3.1 on any card where it loads; it needs DLSS or FSR 3.1
   upscaling on, and the frame limit then counts the frames on screen; with no limit chosen it is the display's refresh
   rate), HDR (the game's picture tone mapped to your display, with its own grading rows, or Native, the game's own HDR; turn HDR on in Windows first) and the
@@ -162,7 +164,8 @@ beside the game.
   the keys `nx2navi.ini` binds when the build runs: after rebinding keys, run
   `py -I tools\make_game.py --rebuild --out "C:\Games\Ocarina"` to draw them again (it skips everything else).
 - The build copies NVIDIA's DLSS library `nvngx_dlss.dll` (with `nvngx_dlss-NOTICE.txt`, which names its licence)
-  and FSR 3.1's `amd_fidelityfx_vk.dll` beside `ocarina.exe` from the kit's `bin\`. To use another DLSS version, put
+  and FSR 3.1's `amd_fidelityfx_vk.dll` beside `ocarina.exe` from the kit's `bin\`, and FSR 4's
+  `amd_fidelityfx_loader_vk.dll` and `amd_fidelityfx_upscaler_vk.dll` when the kit has both. To use another DLSS version, put
   your own `nvngx_dlss.dll` there: later builds keep it. On Linux the library is `libnvidia-ngx-dlss.so.<version>`
   (see "Linux").
 - A kit with `bin\dlssg\` (made with `tools/make_kit.py --dlssg`) also brings DLSS Frame Generation on GeForce RTX 40