feat(setup): check host versions before the build and the runtime install

setup failures reported in #137, #138 and #143 happened only after the long
fetch/build or at Riot startup. scripts/preflight.py now runs from
scripts/build.sh (before configure.py, so an old interpreter gets this message
and not a traceback) and from scripts/install-runtime.sh, and checks the host
once:

- python 3.10+, cmake 3.20+ (the bundled indium CMakeLists requires it),
  git-lfs 2.13+, kernel 5.14+ and LLVM 16+ exit nonzero with one actionable
  line each. RCLOL_SKIP_PREFLIGHT=1 bypasses the gate.
- llvm-objdump and llvm-nm are required, and unresolved names are explained.
- Vulkan 1.3 and timeline semaphores are required when vulkaninfo is
  installed; they only warn when it is not, because a missing tool does not
  mean a missing driver.
- the X display is probed with xdpyinfo in the environment Darling inherits.
  It only warns: a build does not need a display.
- a prepared build/darling-src is scanned for small .dylib/.a/.so/.o files
  that still contain Git LFS pointer text, which links but crashes the guest.
- --json prints the same report for the diagnostic tools.

The installers no longer call hashlib.file_digest (Python 3.11+), so Ubuntu
22.04's Python 3.10 works instead of failing after the overlays are built
(#137). A streaming tools/file_digest.py replaces it.

LLVM command line tools are resolved by tools/llvm_tools.py: an unversioned
name on PATH first, then the highest llvm-<tool>-<N>, then llvm-config
--bindir. Debian/Ubuntu install llvm-objdump and llvm-nm only under
/usr/lib/llvm-<N>/bin, which broke the runtime validation (#138). Six tools
now use it. The LLVM 16 floor also stops the LLVM 15 runtime abort from #139
before anything is built.

Two test helpers used APIs added after Python 3.10; they are shimmed so the
same suite runs on 3.10, 3.11 and 3.14.

Adds tests/test_preflight.py (24 tests, fake host tools only).

docs/SETUP.md

@@ -55,6 +55,8 @@ Linux 5.14 or newer is required for `MADV_POPULATE_WRITE`, used when Darwin's al
 
 The runtime installer requires native `pkg-config` entries for `x11`, `x11-xcb`, `xcb-shm` and `xext`, plus the base Darling SDK.
 
+`scripts/build.sh` and `scripts/install-runtime.sh` start with `scripts/preflight.py`, which checks the host versions the build and the runtime need: Python, CMake, Git LFS, the kernel, LLVM and its command line tools, the Vulkan API level, and the X display. A required check that fails stops immediately with one actionable line; the display check only warns, because a build does not need one. Run it directly with `python3 scripts/preflight.py` (`--json` for a machine-readable report), and set `RCLOL_SKIP_PREFLIGHT=1` to bypass the gate for one run.
+
 Run configuration, builds and the per-user runtime installation as your ordinary account. Use sudo only for the base CMake installation and the incremental server installer. Running the whole build or per-user install under sudo can create root-owned files and configure the wrong guest home. `scripts/install-runtime.sh` initializes the default user prefix and creates `~/.darling/private/tmp` with mode `1777`, so the installer can write guest temporary files. It then shuts down Darling before replacing the paired runtime: first exit the game and both clients manually.
 
 The normal launcher starts neither an Electron inspector nor client API/UI helpers. It clears inherited `DYLD_INSERT_LIBRARIES`, retains Darling runtime library/framework overrides, supplies an absolute log directory to the launcher, and selects the League product. The log-directory/Chromium argument shim, empty-autofill hook and optional native VPX bridge are no longer injected or built by runtime installation. Existing copies on disk are inactive and may remain. Child-process log-directory rewriting and Chromium feature overrides previously supplied by the shim are removed; client startup without those workarounds needs manual validation. The first-time installer uses `--skip-to-install` and omits the installed client's `--app-root`, avoiding an immediate exit or inaccessible intro screen. The project ships no inspector hooks, client API helpers, input automation or client binary patches. Keep account data, logs and captures private.

docs/TROUBLESHOOTING.md

@@ -20,6 +20,29 @@ never delete a populated Riot/game prefix just to repair the launcher.
 
 Use the [setup instructions](SETUP.md) from the repository root as your ordinary user. Exit the game and both clients manually before a full installation or Darling restart. Incremental server/memory installers can replace files without interrupting processes, but their changes need a later full restart to activate.
 
+## Setup stops at preflight
+
+`scripts/build.sh` and `scripts/install-runtime.sh` now run `scripts/preflight.py` before the source fetch, the build and the per-user install. Host versions that cannot build or run the result fail in seconds with one actionable line instead of after the long steps ([#137](https://github.com/lucannez64/darling-lol/issues/137), [#138](https://github.com/lucannez64/darling-lol/issues/138), [#143](https://github.com/lucannez64/darling-lol/issues/143)):
+
+- **python** 3.10 or newer. The per-user installers hash files with a streaming helper instead of `hashlib.file_digest`, which only exists from Python 3.11, so the Ubuntu 22.04 default no longer fails at the end of the install.
+- **cmake** 3.20 or newer. The bundled indium CMakeLists requires it.
+- **git-lfs** 2.13 or newer. The pinned submodules ship libraries as LFS objects.
+- **lfs-pointer**: when `build/darling-src` is already prepared, small `.dylib`, `.a`, `.so` and `.o` files are checked for Git LFS pointer text. A pointer file links but crashes the guest, so this is a required check rather than a warning. It passes before the first fetch, when there is nothing to inspect yet.
+- **kernel** 5.14 or newer for `MADV_POPULATE_WRITE`.
+- **llvm** 16 or newer; 22 or 23 is what the patches are tested with. Iridium calls `LLVMGetVersion`, which LLVM 15 does not export, so an LLVM 15 runtime aborts as soon as Riot creates a Metal device ([#139](https://github.com/lucannez64/darling-lol/issues/139)). LLVM 16-21 builds with a warning.
+- **llvm-tools**: `llvm-objdump` and `llvm-nm`. They are now resolved as an unversioned name, as `llvm-<tool>-<N>`, or from `llvm-config --bindir`, so Ubuntu's `llvm-15-dev` no longer needs symlinks in `/usr/local/bin`.
+- **vulkan** API 1.3 and timeline semaphores. Indium rejects older devices, so this stops the setup with the version `vulkaninfo` reports. Without `vulkaninfo` it only warns, because the tool may be missing even when the driver is fine.
+- **x11** only warns: `DISPLAY` is probed with `xdpyinfo`, and the GL/EGL vendor is reported when a probe is installed. An inaccessible display is what made `iokitd` restart in a loop before [#140](https://github.com/lucannez64/darling-lol/issues/140); export `XAUTHORITY` from servers and SSH sessions. A build without a display is still valid.
+
+Run the same checks by hand, from the checkout:
+
+```sh
+python3 scripts/preflight.py
+python3 scripts/preflight.py --json
+```
+
+If a required check is stricter than your setup, `RCLOL_SKIP_PREFLIGHT=1 ./scripts/build.sh` (and the same for `scripts/install-runtime.sh`) skips the gate for that run. Include the preflight output when reporting a setup failure, and keep `--json` output when a tool needs to parse it.
+
 ## Memory installer: permission denied
 
 An earlier sudo server installation can create root-owned `build/backups`. The old memory installer tried to create `build/backups/memory-runtime` there and failed at `backup_folder.mkdir(...)`. The corrected default is a separate user-owned `build/memory-runtime-backups` directory: