feat: riscv-virt-rt 0.1.0 — board support for QEMU's RISC-V virt machine

[Sunrisepeak]

The first bare-metal package in this index. With it, a firmware project is an
ordinary mcpp project:

mcpp add riscv-virt-rt@0.1.0
mcpp build --target riscv64-none-elf     # + .bin, .map, a size summary
mcpp run   --target riscv64-none-elf     # boots in qemu
mcpp test  --target riscv64-none-elf

and its manifest names no linker script, load address, libc, -nostdlib or
emulator — there is no [target.*] section at all. Verified end to end against
this tree as a path index: added, downloaded, built and booted, printing from
the firmware.

Two descriptor choices are load-bearing, both measured rather than assumed:

• Form A (mcpp = "*/mcpp.toml"), because the package ships a build.mcpp and
  mcpp looks for that at the package ROOT. Form B leaves the root at the
  extract dir and absorbs the tarball's wrap layer inside each source glob,
  which would leave build.mcpp one level down and unfound — the package would
  resolve and compile, then link against nothing.

• deps at the xpm PLATFORM level, carrying xim:picolibc-riscv and
  xim:qemu-riscv. mcpp materializes [xlings] deps for the ROOT project only,
  so the package's own declaration does not reach a consumer. Verified by
  removing picolibc from the store: mcpp add + mcpp build reinstalled it
  and linked; without this edge the build stops at build.mcpp saying the
  sysroot is declared but not installed.

Requires mcpp >= 2026.8.19.2 (build.mcpp calls mcpp::runner). No workspace
member under tests/examples/: those run on all three platforms with no
capability gate, and this one needs an emulator and a target sysroot. The
package's own repo CI installs both and boots rv64 and rv32 on every push.