fix: spell every ${mcpp.*} path token generically (#393)

[ZheFeng7110]

Summary

• Spell every ${mcpp.*} path token with forward slashes. Five sites in
  src/build/prepare/plan.cpp replaced a path with its native .string()
  while ${mcpp.target_file:} and ${mcpp.artifact:} were already
  .generic_string(), so the one closed vocabulary had two separator
  conventions.
• A build program that wrote ${mcpp.out_dir}/gen/x.cppm therefore got
  C:\proj\target\…\gen/x.cppm on Windows: one argv token with a native prefix
  beside the author's / (fix: build action 的 ${mcpp.out_dir} 替换把原生拼写拼进用户正斜杠路径,产出混合分隔符 #393). ${mcpp.bin_dir}, ${mcpp.compile_db},
  ${mcpp.self} and ${mcpp.stage_dir} had the same shape.
• The five sites now use .generic_string(), the spelling escape_ninja_path
  already gives every ninja node name and every tool path (the compiler, ar,
  mcpp itself), so the vocabulary agrees with the rest of the graph. POSIX
  output is byte-identical, so no existing edge changes there.
• docs/30-build-mcpp.md and docs/zh/30-build-mcpp.md state the rule beside
  the vocabulary table; CHANGELOG.md records it.

Note on the issue's suggested fix: make_preferred on the replacement value is
a no-op here — ctx.plan.outputDir is built with operator/ throughout, so it
is already natively spelled. The mixed token comes from joining a native prefix
with the author's /; generic is what makes the two agree.

Test plan

• mcpp test with the freshly built binary: 150 passed; 0 failed
• tests/e2e/188_build_actions.sh with the freshly built MCPP (it drives
  ${mcpp.out_dir}, ${mcpp.bin_dir} and ${mcpp.target_file:} end to end)
• tests/e2e/660_an_action_names_the_engine_through_mcpp_self.sh with the
  freshly built MCPP; it now also asserts the action rule's argv carries no
  backslash, which the windows-e2e shard this token exists for is what
  exercises (on POSIX the two spellings are equal, so that leg is a no-op)
• bash .github/tools/check_docs_style.sh

Closes #393.