Implement the typed doctest core architecture

[tony]

Depends on #92.

Summary

• Implement the proposed typed, host-neutral doctest_core pipeline for parsing, projection, registration, and execution.
• Refactor doctest_docutils and pytest_doctest_docutils as adapters over the core while preserving their command and collection surfaces.
• Keep pytest shared-state groups schedulable as one item while retaining an independent doctest.DocTest and source location for each block.
• Expand compatibility coverage and pin the CI matrix to exact supported docutils, pytest, pytest-asyncio, and pytest-rerunfailures versions.

The architecture ADR remains proposed. This draft implements it against the existing direct and pytest entry points so the contracts can be reviewed against executable behavior.

Why

The direct and pytest paths previously owned overlapping parsing and execution behavior. The pytest adapter also blocked the standard doctest plugin while depending on its implementation. A shared core separates host integration, scheduling identity, and diagnostic identity without merging source blocks or adding an xdist affinity protocol.

Impact

Documentation blocks within a group share one namespace, but each block retains its own location and failure detail. The pytest adapter coexists with the standard doctest plugin, so pytest continues to provide fixtures, checker and report options, and Python-module doctest collection.

Validation

• uv run ruff format . --check
• uv run py.test
• uv run ruff check .
• uv run mypy .
• just build-docs

Known CI failure

The push workflow fails in mypy for Python 3.13 with docutils==0.21.2 and pytest==7.2.0. An isolated local run with the same pins reproduces strict typing errors in src/_pytest_doctest_compat.py and src/pytest_doctest_docutils.py; the equivalent pytest==8.4.2 run passes. The oldest pytest line exposes the private compatibility surface with weaker annotations, so the blocker remains unresolved.