feat(toolchain): resolve the compiler in one place

Every clang++, clang and llvm-ar the tool spawned was a bare name on
PATH, the std module source was /usr/share/libc++/v1/std.cppm, the WASI
sysroot /usr/share/wasi-sysroot and the mingw headers
/usr/x86_64-w64-mingw32, spelled out at eighteen call sites. A
toolchain installed anywhere else, which is what the Flatpak ships
under /app, had no way in.

Crafter.Build:Toolchain resolves all of it once per process:
CRAFTER_BUILD_LLVM_DIR, else a clang++ installed beside crafter-build
(<prefix>/bin, found through the same walk GetCrafterBuildHome does),
else PATH. What sits beside the compiler is used when present: its
libclang.so, its clang-c headers, its std module source, its WASI
sysroot. An explicit prefix without a clang++ is an error rather than a
fallback, for the same reason CRAFTER_BUILD_LIBCLANG was already
exclusive. CRAFTER_BUILD_LIBCXX_DIR points at a libc++ install other
than the compiler's own, and CRAFTER_MINGW_DIR now does what its help
line always claimed: it is passed as --sysroot for the mingw target.
Binaries linked by a toolchain outside /usr get an rpath to its lib
dir, since its libc++.so.1 is not on the loader's default path.

HostTarget() is the resolved clang's -print-target-triple. A clang built
without a vendor reports x86_64-unknown-linux-gnu where Arch's reports
x86_64-pc-linux-gnu, so target equality that decides whether a test runs
locally and which targets a bare `test` sweeps goes through SameTarget,
which ignores the vendor field. The strings themselves are never
rewritten: clang rejects a PCM whose triple differs from the importing
unit's. The examples and README stop hardcoding the host triple; the
default is already the host.

`crafter-build --version` prints the version baked in at build time
(CRAFTER_BUILD_VERSION, else `git describe`, else 0.0.0-dev), the
compiler and the host triple, and is answered before any argument is
forwarded to project.cpp. The version is a define on the library, so
each version is its own library variant and relinks the executable and
tests once per commit.

build.sh gains a shebang and `set -eu`, honours CRAFTER_BUILD_LLVM_DIR
and CRAFTER_BUILD_VERSION, skips the glslang clone when it is already
there, and writes a fresh archive instead of appending to one that may
hold members from an older layout.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

interfaces/Crafter.Build-Clang.cppm

@@ -355,12 +355,21 @@ export namespace Crafter {
         bool needsAst = false;
     };
 
-    // The host target triple, detected once per process by running
-    // `clang++ -print-target-triple` and cached. Used as the default for
+    // The host target triple: what the resolved toolchain's clang++ prints
+    // for -print-target-triple (see GetToolchain). Used as the default for
     // Configuration::target so projects don't have to hardcode it for the
-    // no-cross-compile case. Returns "" if clang isn't on PATH or doesn't
-    // print a triple — those projects still need an explicit cfg.target.
+    // no-cross-compile case. Returns "" when no compiler could be resolved;
+    // those projects still need an explicit cfg.target, and the first build
+    // step reports the missing compiler.
     std::string HostTarget();
+    // Target equality that ignores the vendor field: x86_64-pc-linux-gnu,
+    // x86_64-unknown-linux-gnu and x86_64-linux-gnu are one target as far as
+    // "does this test run on this host" goes, and which spelling a clang
+    // reports depends on how it was built. The strings themselves are never
+    // rewritten: clang rejects a PCM whose triple string differs from the
+    // importing unit's, so what the user wrote is what the compiler sees.
+    std::string NormalizeTarget(std::string_view triple);
+    bool SameTarget(std::string_view a, std::string_view b);
 
     struct Configuration {
         fs::path path;