feat(packaging): ship crafter-build as a Flatpak with its own toolchain

packaging/flatpak/ holds the manifest for net.catcrafts.CrafterBuild and
its side files. The runtime is the Freedesktop Sdk, not the Platform: a
build tool needs glibc's headers and crt objects, gcc's libgcc_s, cmake,
ninja, git and python at run time. Everything compiler-shaped is ours
under /app, built from one LLVM 22.1.8 source tree: clang, lld, libclang,
libc++ with its std module, compiler-rt for the host and for wasm32, the
WASI sysroot (wasi-sdk 34 in sysroot-only mode, driven by the clang just
built) and wasmtime for running the result. One tree means the modules
crafter-build builds and the compiler that consumes them cannot drift
apart, and the toolchain resolver finds all of it beside the executable
with no configuration.

The crafter-build module runs the bootstrap, the self-host build and the
test suite inside the sandbox, so a Flatpak that installs has passed the
suite with the toolchain it ships. CRAFTER_BUILD_MARCH is x86-64-v2 for
the shipped binary; user projects keep -march=native.

WasmHelloRuns compiles a fixture for wasm32-wasip1 and runs it under
wasmtime. The existing wasm tests only inspect generated files, so this
is the one that proves the sysroot, the std module for wasm and wasm-ld
work together; it skips where the sysroot or wasmtime is missing.

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

packaging/flatpak/catcrafts.flatpakrepo

@@ -0,0 +1,8 @@
+[Flatpak Repo]
+Title=Catcrafts
+Url=https://flatpak.catcrafts.net/repo/
+Homepage=https://forgejo.catcrafts.net/Catcrafts
+Comment=Catcrafts tools and applications
+Description=crafter-build and other Catcrafts software, built and signed by Catcrafts CI
+DefaultBranch=stable
+GPGKey=GPG_KEY_BASE64_PLACEHOLDER