docs: describe how a sysroot build finds its std module source

The README said a cross build precompiles the sysroot's own std.cppm,
which was never the whole story for sysroots that do not ship one. It
now states the order (the sysroot's, the toolchain's for the same major
release, the sources crafter-build carries for other releases), what
the failure looks like when nothing matches and how to resolve it, and
lists the new directory in the install layout.

AGENTS.md records the rule for the next LLVM bump: the release the
bundled toolchain leaves behind stays available as a module-source
module, because stable distros remain on it long after.

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

README.md

@@ -116,6 +116,8 @@ Inside the Flatpak:
 /app/bin/clang++ lld llvm-ar wasmtime    # the bundled toolchain the executable resolves first
 /app/lib/libcrafter-build.a              # static archive for downstream consumers
 /app/share/crafter-build/*.cppm          # module sources (rebuilt on the user's machine on first run)
+/app/share/crafter-build/libc++-modules/<major>/
+                                         # std module sources of other libc++ releases, for sysroots without their own
 /app/share/wasi-sysroot/                 # WASI sysroot for wasm32 targets
 ~/.var/app/net.catcrafts.CrafterBuild/cache/crafter.build/<target>-<march>-<clang>-<hash>/
                                          # std.pcm + Crafter.Build*.pcm, built locally per toolchain
@@ -174,7 +176,7 @@ TestRunner::Cmd("wine")                              // local exec via prefix co
 
 For elaborate setups (rsync, ssh ControlMaster, env vars on the spawn, `QEMU_LD_PREFIX` for sysroots), construct `TestRunner` directly — `copy`/`exec`/`cleanup`/`probe` are public string fields with `{bundle}`, `{remote_bundle}`, `{bin}`, `{bin_name}`, `{args}` placeholders.
 
-For cross-arch builds, set `cfg.sysroot` on the `Configuration` to point at a sysroot for the target. `BuildStdPcm` and the compile/link commands both consume it: `--sysroot=<path>` is passed to clang, and `<sysroot>/usr/share/libc++/v1/std.cppm` is precompiled instead of the host's. The cheapest aarch64 sysroot is an extracted [Arch Linux ARM rootfs](http://os.archlinuxarm.org/) plus `libc++` + `libc++abi` + `gcc` installed into it via `pacman --root <rootfs> --config <alarm-pacman.conf> --dbpath <rootfs>/var/lib/pacman -Sy …` (point pacman at ALARM's mirrors via explicit config to avoid pulling host-arch packages).
+For cross-arch builds, set `cfg.sysroot` on the `Configuration` to point at a sysroot for the target. `BuildStdPcm` and the compile/link commands both consume it: `--sysroot=<path>` is passed to clang, and the std module is precompiled against the sysroot's libc++ headers. The module source has to come from the same libc++ release as those headers, so it is taken from `<sysroot>/usr/share/libc++/v1/std.cppm` when the sysroot ships it, else from the toolchain when both are the same major release, else from the sources crafter-build carries for other releases (`share/crafter-build/libc++-modules/<major>/`; `crafter-build --version` lists them). Alpine is the common case for the last two: its `libc++-dev` has the headers and no module source. When nothing matches, the build stops with one line naming both releases; installing that release's `share/libc++/v1` directory into the sysroot resolves it. The cheapest aarch64 sysroot is an extracted [Arch Linux ARM rootfs](http://os.archlinuxarm.org/) plus `libc++` + `libc++abi` + `gcc` installed into it via `pacman --root <rootfs> --config <alarm-pacman.conf> --dbpath <rootfs>/var/lib/pacman -Sy …` (point pacman at ALARM's mirrors via explicit config to avoid pulling host-arch packages).
 
 For `--target=x86_64-w64-mingw32` (Windows from Linux), the pipeline cross-compiles as long as `mingw-w64` is installed. Pair it with `--runner=cmd:wine` to run the result. Only the MinGW ABI is supported; there is no MSVC target and no Windows host. `-fuse-ld=lld` is auto-injected on every link, and `-lstdc++exp` is auto-linked for the mingw target so user projects don't need to declare those flags.