feat: declare minimum compatible version for typing-extensions - #214

Part of #213.

Core's only runtime dependency was declared bare. A bare install resolved at its
floor gets typing-extensions==3.6.2 and does not import:

ImportError: cannot import name 'GenericMeta' from 'typing'
  at lite_bootstrap/instruments/base.py:4

ADR-0007
argues this dependency is load-bearing and names both uses; the floor those uses
imply was never written down.

typing-extensions>=4.6

How the floor was established

Bare core (uv pip install --no-deps .) plus one pinned typing-extensions,
import lite_bootstrap, on every interpreter in the CI matrix:

Two independent constraints, so the comment follows the litestar>=2.15 idiom of
naming both and letting the higher one win:

• Below 4.0: AttributeError: module 'typing_extensions' has no attribute 'Self',
  raised while evaluating the return annotations on BaseConfig.from_dict and
  from_object.
• 4.4.0 and 4.5.0: TypeError: type 'typing.TypeVar' is not an acceptable base type, raised inside typing_extensions itself on Python >=3.12, which made
  typing.TypeVar non-subclassable. Neither release declares a Requires-Python
  ceiling, so a resolver on 3.12+ will pick them under a >=4.0 floor. That is
  what rules out the simpler declaration.

After the change, uv pip install --resolution lowest-direct . resolves
typing-extensions==4.6.0 and imports clean on 3.10 through 3.14.

The 3.14 column

3.10.0.2 passes on 3.14 alone because PEP 649 defers annotation evaluation there,
so the missing Self is never looked up at import. The TypedDict base class is
still evaluated eagerly and 3.10.0.2 provides it. The floor is set by the
interpreters where the annotation is evaluated, not by the one where it is not.

What this does not do

No test accompanies the declaration, matching #211: the claim is about resolution,
which nothing in the suite exercises. just test-ci (277 passed, 100% coverage)
and just lint-ci pass unchanged, and uv.lock is untouched because every extra
already pulls typing-extensions far above 4.6 transitively. #210 is the job that
would hold this floor; until it exists the declaration is verified by the table
above and nothing else.

Eleven bare requirements remain, tracked in #213.