The source_viewer component used static DOM IDs (id='source-viewer')
with phx-update='ignore'. When switching expanded files, LiveView
reused the same DOM element but phx-update='ignore' prevented content
from being updated, showing the previous file's source.
- Override source_viewer/1 in MicroprintsLive with unique per-file IDs
generated via :erlang.phash2(file_path)
- Add test verifying expand-switch shows correct source content
- Add test for highlight/expand coupling (collapse when highlighting
a different file)
- All 160 tests pass
