On failure the error window showed a generic message with the whole log
buried in an expander. Lead instead with a focused summary: which step
failed (the process label), its command, and the last stderr lines, while
keeping the full log below. The runner passes the failing process to
error_window, which reads the executor's bounded stderr_data.
build_summary/_stderr_tail are static and unit-tested (tests/
test_error_summary.py): tail extraction, CR handling, blank-line skipping,
stdout fallback, long-line truncation, and Pango-markup escaping of paths/
messages containing & < ' so set_markup can't choke.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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