Fail CI when regenerated sources differ from the committed ones

[MisterGC]

The sources-regeneration step in build_linux.yml and build_windows.yml now always regenerates and goes red when the result differs from the committed src.

• zserio runs with -ignoreTimestamps: without it zserio skips writing files that look newer than tutorial.zs, so the step could pass without regenerating anything
• after generation, git add --intent-to-add src + git diff --exit-code -- src fails the step on any change, including a generated file that is not committed
• keep-alive needed no change: both workflows already run their own keepalive job (liskin/gh-workflow-keepalive@v1, added in 1e0639b), which re-enables the workflow it runs in

Verified:

• keep-alive (criterion 1): gh run view of the latest scheduled runs, Linux 30725731727 and Windows 37166081740 → keepalive success in both; gh api .../actions/workflows → both active
• regeneration fails on a diff (criterion 2), zserio 2.19.0 under bash -eo pipefail in a clone: clean tree → exit 0; a committed file with an extra line → exit 1; a generated file missing from the repo → exit 1
• the same three cases without -ignoreTimestamps: the edited file → exit 0 (zserio skipped regeneration), which is why the flag is in
• all Linux workflow steps run locally (runtime + zserio download, regenerate, diff check, javac, Main write_joe/read/write_boss/read) → exit 0
• PR CI: build_linux pass (run 37275727859), build_windows pass (run 37275727854, log shows the regeneration and git diff --exit-code ran)
• 2 files changed, 6 insertions(+), 2 deletions(-)

Not verified: the red path on the Windows runner (only shown locally on macOS); a scheduled run of the changed workflows. No verify.sh or test suite exists in this repo.

Closes #2