docs(roadmap): M4 requires only what phase 4 builds, and T87 owns the dry run

[haiquang9994]

M4 required mix uninstall --dry-run, and no task in phase 4 builds mix uninstall — the complete uninstall path is T87, in phase 9. So the milestone could not be reached at the end of its own phase however much of that phase was done, and todo.md had already been stating M4 without the clause: the index was right and the phase file was wrong.

Three documents corrected as one change, because a milestone fixed in one file and contradicted in another is not fixed:

• phase 4 — M4 drops the clause, and says why it went, so it does not get added back. What the clause was reaching for is kept and given owners: phase 4 is where MixEngine first touches the machine outside its own home (hosts block, port grant, resolver wiring, audit log), enumerating that is T47's, and removing it is T87's.
• phase 9 — T87 gains --dry-run explicitly, with what it must list, and a note that it reads T47's inventory rather than building a second one.
• build-and-release.md — removing the helper's audit log was attributed to "(T47, T92)". Both wrong: T92 is the public beta, and T47 is a diagnostic. It is T87's, and mix doctor reports the log without removing it — a diagnostic that deleted a root-owned audit trail would be deleting the record of what it was diagnosing.

No CI: this touches only markdown, and none of ci.yml's four jobs reads it.