fix: __version__ from metadata; revert the fastmcp pin (v0.5.2)

[Roy-Kid]

Two corrections to 0.5.1.

__version__ disagreed with the distribution

0.5.1 shipped like this:

dist version      : 0.5.1
molmcp.__version__: 0.5.0

The literal in __init__.py sat beside the real version in pyproject.toml,
and a release bumps the manifest. It reads importlib.metadata.version now —
one source of truth — and three tests fail if a literal comes back.

The 0.5.1 pin did not do what it was meant to

fastmcp==4.0.0b2 was supposed to let uv install molmcp without
--prerelease=allow. Measured side by side, it does not:

uv pip install dist/*.whl                    → + fastmcp==4.0.0b2   ✅
uv pip install "molcrafts-molmcp==0.5.1"     → ✗ "there is no version of fastmcp==4.0.0b2"

A local wheel resolves either way; from an index uv refuses the exact pin
for the same reason it refuses the range — it does not enable pre-releases for
a transitive dependency, however that dependency is spelled. The pin bought
nothing and cost the automatic upgrade, so this reverts to >=4.0.0b1,<5:
resolves to 4.0.0b2 today, picks up 4.0.0 the day it ships, and the uv problem
goes with it.

Still FastMCP 4 only. 3.x speaks the protocol this repo was rebuilt off,
and a bare fastmcp lands on 3.4.6. >=4.0.0 resolves to nothing — PyPI's
4.x line is a1/a2/b1/b2 with no final yet.

Until then the flag is documented rather than worked around:
installation.md explains why uv add --prerelease=allow is needed, that pip
needs nothing extra, and that pinning is not a fix. The migration guide points
at it.

Test plan

• pytest — 1028 passed, 2 skipped (3 new version-consistency tests)
• ruff check / ruff format --check clean
• __version__ and the distribution version agree
• Every internal doc link resolves
• CI green