chore: 🔧 enforce type checking with ty (fixes #16)

[pgrobasillobre]

🤖 AI text below 🤖

Adopts ty as the type checker, replacing the dead [tool.pyright]
block (never invoked anywhere) with a working [tool.ty] config,
wired into prek.toml. Keeps the toolchain consistent with the
project's existing uv/ruff setup.

Silences the expected unresolved-import on the optional cupy
dependency, tightens dtype: type → numpy.typing.DTypeLike (6x),
and gives the bare NDArray in utils/linalg.py its generic
parameter. Also fixes a genuine, separate deprecation found along the
way in logging_config.py.

Note: the issue mentions .pre-commit-config.yaml, but this repo
uses prek.toml — the ty hook was added there instead. No change
needed in lint.yml, since it already runs prek.

Known, out-of-scope finding: ty check surfaced 8 more
diagnostics beyond the cupy one mentioned in the issue — eta = [None] * n_sites filled via a loop, which ty's static analysis
can't fully verify across a dynamic loop range, though the code and
tests are correct. Suppressed with targeted ignores; a proper fix
(local-variable capture + an assert before the final return) is a
good follow-up, but touches working numerical code and felt out of
scope here.

Local-only note: prek run --all-files and uv sync --all-groups --all-extras both fail here (macOS/Apple Silicon) on
cupy-cuda12x, which has no arm64 wheel. Confirmed this doesn't
affect CI, which runs on ubuntu-latest.

ty vs mypy: went with ty for toolchain consistency,
understanding it's pre-1.0 (per the issue's own caveat). Happy to
switch if preferred.

Testing: uv run ty check src/, uv run ruff check src/ tests/,
uv run ruff format --check src/ tests/, uv run pytest -m "not slow",
and uv run pytest (full suite) all pass locally.

AI/LLM disclosure

• I did not use LLM tooling, or used it only privately for ideation
• Used to help write this PR description: Claude (claude.ai, Sonnet 5)
• Used to generate or modify code: Claude (claude.ai, Sonnet 5) — suggested specific code changes throughout; I applied every edit myself and verified each one (ty check, tests) before committing