fix: route explicit ninja goals through the manifest, not the command line — 0.0.108

[Sunrisepeak]

mcpp test names every shared prerequisite as a ninja goal so a broken package
source fails once, as a package error, instead of N times as identical per-test
compile failures. For a large package that is thousands of object paths:
FFmpeg's 2281 translation units produced a 50,781-character argv. Windows joins
argv into a single command string for cmd.exe, which caps at 8191 characters, so
the command never ran at all — the 127 came back from cmd.exe, before ninja or
mcpp could print anything.

That silence is what made it hard to place. mcpp-index's Windows job has failed
on the ffmpeg member since 0.0.104, and the log's last line was an unrelated
download progress bar: no "build failed" diagnostic, no ninja output, and
target/.build_cache never written. ffmpeg is the only member large enough to
trip the limit, and mcpp build / mcpp run drive default with no goal
arguments, so both stayed green throughout.

Emit the goal set as a phony edge in build.ninja and pass its one-word name. The
manifest has no length limit, and ninja still builds the whole set in a single
invocation, so parallelism is unchanged.

Same origin as the soname-alias regression fixed in 0.0.107: both come from the
explicit-goal-targets change in #274 — one design change, two symptoms that only
appear under conditions the test suite did not cover.

e2e 164 asserts the mechanism (goals live in the manifest) rather than the 8191
limit: reproducing the limit needs a package far too slow to build here, and the
limit does not exist on Linux or macOS. It is red on 0.0.107 and green here.

Verified: unit 37/37; e2e 155 passed / 1 failed / 5 skipped, the one failure
(22_doctor_cache_publish) reproducing identically on the released binary under
the same MCPP_HOME.

Found by publishing an instrumented Windows build to a throwaway prerelease and
pinning it from a stripped-down mcpp-index probe PR — four rounds of black-box
probing could not see inside the process.