feat: std-freestanding 0.2.0 + riscv-virt-rt 0.3.0 — the target owns its C library - #220

…d library

`import std;` is one module over the whole library, so there is no subset of IT
to build without an OS — mcpp turns it off on a freestanding target and says
so. But libc++'s HEADERS are almost entirely freestanding-capable already; what
stops them is one per-target file, `__config_site`, which the toolchain ships
only for its own host triple. Synthesising that file is this package's job.

103 of libc++'s 110 headers compile for riscv64-none-elf. The other 7
(generator, hazard_pointer, rcu, spanstream, stacktrace, stdfloat,
text_encoding) fail on an x86_64 host with full libc++ and glibc too — they are
headers libc++ has not implemented, so the freestanding loss at compile time is
zero. The package's regeneration script prints that control group on every run.

Verified running under qemu: array, span, optional, atomic, string_view, and
ranges::sort with a projection.

`xim:llvm` is an install-time dependency because that is where libc++'s headers
live. Nothing is linked from it — the package includes them privately and
exports a module — but without the payload there is nothing to include, and the
failure would surface as "file not found" inside a module compile instead of as
a missing dependency.

Requires mcpp >= 2026.8.19.4: earlier versions do not put `-fno-exceptions` on
a freestanding target, and `std::optional::value()` alone then pulls in
`__cxa_throw` and three more symbols that cannot exist without an unwinder.

…its C library

Both packages declared `xim:picolibc-riscv@1.8.12`, and the standard-library
subset also declared `xim:llvm`. That pinned a board package and an
implementation-neutral library alike to one C library, one architecture, one
toolchain and one version of each — none of which is a property of either.

mcpp 2026.8.19.4 resolves the target's C library from the target's own row, the
way it resolves the compiler: a hosted project never declared glibc, and a
bare-metal one no longer declares picolibc. So:

  * std-freestanding 0.2.0 declares NOTHING. It asks mcpp where the toolchain's
    headers are, and the target's C headers are already on the compile line.
  * riscv-virt-rt 0.3.0 declares only the emulator, which is the one xim
    package that really is a board fact. Its linker line is bare names now,
    because the search path is there before it says anything.

riscv-virt-rt 0.1.0 and 0.2.0 stay listed — published versions someone may have
pinned. std-freestanding 0.1.0 is replaced rather than kept: it never reached
this index, and it is wrong by construction.

Both new versions require mcpp >= 2026.8.19.4.