Cross-platform QA: Windows + Linux CI matrix

[rosshd]

Objective

Verify openlearn on Windows and Linux before marketing it; development happens on macOS.

Scope

• Add an OS matrix (ubuntu, windows, macos) to the tests workflow.
• Fix what breaks; known risk areas: `select.select` on stdin in the REPL paste heuristics, TTY assumptions, path handling, Rich rendering differences.
• Document any platform caveats that remain.

Verification

• Green CI on all three OSes.
• Manual smoke of the REPL on Windows Terminal and a mainstream Linux terminal.

[rosshd]

Board hygiene status: the cross-platform CI matrix is implemented on origin/main. The issue remains open because its acceptance criteria also require documented manual REPL smoke runs in Windows Terminal and a mainstream Linux terminal.

[rosshd]

Automated cross-platform acceptance is green on merged main commit 547511a2aacc9b67355ca4c1741cf03a00cb1a0f.

• CI: https://github.com/rosshd/openlearn/actions/runs/31354003855
• Tests and immutable candidate: https://github.com/rosshd/openlearn/actions/runs/31354003862
• Source and installed-package lanes passed on Linux, macOS, and Windows with Python 3.11, 3.12, and 3.13.
• Browser smoke, security regressions, the full automated gate, and wheel/sdist clean-install smoke passed.
• PR fix(data): stabilize home inventory snapshots #77 fixed the Ubuntu inventory snapshot race exposed by the prior main run and added cross-process regression coverage.

This issue remains open only for the manual REPL smoke runs in Windows Terminal and a mainstream Linux terminal.