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).

README.md

@@ -41,7 +41,7 @@ Game files, Riot installers, account databases, tokens, local prefixes, captures
 ## Requirements
 
 - x86-64 Linux **5.14 or newer**, an X11 session or Xwayland, and a working hardware Vulkan/OpenGL driver. AMD RADV and the NVIDIA driver (proprietary and open kernel modules) have been exercised, see [NVIDIA](docs/TROUBLESHOOTING.md#nvidia); other drivers are not validated. Hybrid laptops usually render on the integrated GPU unless started with PRIME offload. See [hybrid graphics](docs/TROUBLESHOOTING.md#hybrid-graphics-laptops-amdintel-igpu--nvidia).
-- Git + Git LFS, Bash, curl, unzip, CMake, Make, Clang/LLD, LLVM development files, a C/C++ compiler, Python 3.12+ and the Darling build dependencies. The patch includes LLVM 22/23 branch-opcode compatibility; the new PC used LLVM/Clang 23. Node.js is needed only for the offline JavaScript checks.
+- Git + Git LFS (2.13 or newer), Bash, curl, unzip, CMake 3.20+, Make, Clang/LLD, LLVM development files (16 or newer), a C/C++ compiler, Python 3.10+ and the Darling build dependencies. `scripts/build.sh` and `scripts/install-runtime.sh` run `scripts/preflight.py` first and stop on a version that cannot build or run the result; `RCLOL_SKIP_PREFLIGHT=1` bypasses that gate. The patch includes LLVM 22/23 branch-opcode compatibility, and LLVM 22 or 23 is what the Metal overlays are tested with. Node.js is needed only for the offline JavaScript checks.
 - X11/XCB/SHM development files for the current runtime helpers.
 - Enough free disk for Darling sources/build, Git LFS assets and the game: budget at least 50 GB. Building is substantially larger than this source-only repository.