release: 0.3.0

[fredaime]

The command-line client at 0.3.0, published as one commit: the tree reviewed in
the private repository it is built in. CHANGELOG.md says what changed for a
caller; in short:

• --socket is optional for a per-user profile on every verb, and --pack
  takes the name of a pack this distribution ships as well as a directory.
• A target led by an interpreter, or an executable whose shebang names one,
  runs under that interpreter with the boundary in it; a runner in that place
  is refused.
• instrument run --follow-children installs the boundary in the Python
  children a program starts with its environment.
• instrument verify matches a run's records by a correlation it stamps, asks
  for every effect, counts one spawn once and a process multiprocessing
  starts, and answers 3 for a refused chain read.
• instrument run ends with the outcome's own status when the program lets a
  boundary outcome escape.
• Every connection is bounded, and the suite runs on Python 3.12, 3.13 and 3.14.
• QUICKSTART.md is a transcript of 0.3.0.

It needs sayfirst-contract and sayfirst-boundary 0.3.0, published by the
control plane's repository at its v0.3.0 tag.

The whole gate was run over exactly this tree before it was pushed, against the control plane's public v0.3.0 tag: format, lint, 1067 passed and the dependency-closure check. The tag v0.3.0 is cut on this commit once it is on main and main is green; the release workflow's publish job then waits for the pypi environment's reviewer.