ci: move bench to scheduled workflow + add Python 3.13 (R10, R11)

R10 — `cross-library-bench` previously ran on every push and every PR
to `main`, adding 5–10 minutes of build + bench time per CI run with
no automated consumer of the artefact. It moves to a dedicated
workflow (`.github/workflows/bench.yml`) that fires nightly at 03:00
UTC and on-demand via `workflow_dispatch` (with optional `size` /
`iterations` inputs). The job in `ci.yml` is removed and a pointer
comment is left in its place so future readers find the new home.

R11 — `pyproject.toml`'s `requires-python = ">=3.9"` already allowed
Python 3.13 installs, but the classifier list stopped at 3.12 and CI
tested only 3.9 / 3.11 / 3.12. The Python CI matrix gains `"3.13"`
and the matching `Programming Language :: Python :: 3.13` classifier
is added so PyPI listings and version-search tooling reflect the
actually-tested range.

bindings/python/pyproject.toml

@@ -20,6 +20,7 @@ classifiers = [
     "Programming Language :: Python :: 3.10",
     "Programming Language :: Python :: 3.11",
     "Programming Language :: Python :: 3.12",
+    "Programming Language :: Python :: 3.13",
     "Programming Language :: Rust",
     "Topic :: Office/Business :: Financial :: Investment",
     "Topic :: Scientific/Engineering :: Mathematics",