Add Python 3.15 support to CI and release workflows

[harryswift01]

Summary

This PR adds Python 3.15 to the CI test matrices and moves the single-version CI and release jobs from 3.14 to 3.15, so CodeEntropy is tested and built on the latest Python.

Changes

Test matrices:

• Add "3.15" to the Python version matrix in pr.yaml, daily.yaml and weekly-docs.yaml. The matrices now cover 3.12–3.15 on Ubuntu, macOS and Windows.

Single-version CI jobs:

• Move the docs, pre-commit, and regression/testing jobs in pr.yaml from Python 3.14 to 3.15.
• Move both jobs in weekly-regression.yaml from Python 3.14 to 3.15.
• Move update_regression_baselines.yaml from Python 3.14 to 3.15, so baselines are regenerated on the same version the regression tests run on.

Release workflow:

• In release.yaml, bump the pinned setup-python version from 3.14.8 to 3.15.0 for the version-check and flit build jobs.
• In release.yaml, bump the conda build environment from Python 3.14 to 3.15.

Impact

• Incompatibilities with Python 3.15 will show up in every PR and daily run.
• Releases are built and published with Python 3.15.
• Regression baselines are generated and checked on Python 3.15.
• Python 3.12–3.14 stay in the test matrices, so requires-python = ">=3.12" is unchanged.