fix: a cross target's toolchain section is not the build program's; a test name is not another's directory; CI findings

- Under a cross `--target`, a `[target.<t>] toolchain` names the target's
  compiler; the build program takes the spec it replaced, as under a row's pin
  (mcpp's own tree built on Windows for x86_64-linux-musl compiled its build
  program with the musl cross gcc). e2e 1051.
- A test name that is another's directory (`unit`, `unit/own`) is refused,
  naming both.
- The rows' snapshot keeps LF on every checkout (.gitattributes).
- e2e 298 leaves out x86_64-windows-gnu, whose MinGW-w64 payload llvm pairs
  with; openkal-cross names the openkal macOS identity, aarch64-macos-musl.
- The stray empty `.edited` is removed; e2e 859 names a missing record.

Co-authored-by: speak-agent <248744407+speak-agent@users.noreply.github.com>

Author: Sunrisepeak