Register `mdscript` name on PyPI (pre-tag prerequisite)

[dean0x]

`crates/mds-python/README.md` advertises `pip install mdscript` but the name is currently unregistered on PyPI. An unregistered name is a squatting risk — a third party could claim it before the first wheel is published.

This is the only genuinely pre-tag part of #132 (cross-platform wheel matrix + PyPI publishing). The name reservation should happen before the v0.4.0 tag; the wheel CI wiring is deferred in #132.

Acceptance Criteria

• Log in to PyPI as the package maintainer
• Register the `mdscript` name (e.g., by publishing a minimal placeholder release via `maturin publish` or PyPI's name-reservation mechanism)
• Confirm the name appears at https://pypi.org/project/mdscript/ and is owned by the designated maintainer account
• Update `RELEASING.md` to note the name is registered (prerequisite for Python bindings: cross-platform wheel matrix + PyPI publishing (+ cp314t) #132)