Track Python 3.15 core dependency compatibility

[stefan-jansen]

Engineer currently declares Requires-Python: >=3.12,<3.15 under the ecosystem Python 3.15 waiting policy. Python 3.12 through 3.14 remain the supported release range.

Run a visible, non-blocking Python 3.15 core canary at least monthly. Recheck immediately when Python 3.15 final ships or when Polars publishes a stable release after 1.44.2. Removing the upper bound requires the complete non-hardware-dependent suite, package build, and installed-wheel import to pass on Linux, macOS, and Windows.

This is an external-dependency waiting record, not a blocker for an otherwise qualified 0.1 patch release. Tracked by ml4t/ecosystem#33 and Decision 0013.

[stefan-jansen]

Initial evidence on 2026-09-20:

• Python 3.12-3.14 is green locally: 3,925 tests passed, with the focused public API and bar-sampler contract suite also passing.
• An isolated CPython 3.15.0rc1 run against the current stable core constraints stops during dependency installation because stable Pydantic selects pydantic-core==2.46.5, which has no CPython 3.15 wheel.
• Pull request ci: complete Engineer closure audit #57 adds the monthly Ubuntu canary. It is scheduled and manually dispatchable, but is not a pull-request or release gate.

Keep this issue open. Recheck on Python 3.15 final or after a stable Polars release newer than 1.44.2, then require the full Linux, macOS, and Windows qualification before removing the upper bound.