Problem: a TU the front end could not parse left no trace. ClangTool::run's
status was discarded, TUs with compile errors were analyzed on a partial
AST and counted as done, and the isolated-worker path dropped TUs that
never produced an AST. A scan of only broken files printed no findings and
exited 0, the same as a clean scan. Separately, the scan logger wrote to
stdout, so any warning corrupted a SARIF/JSON report written to stdout.
Fix: every TU now yields an outcome (clean, compile-errors,
analysis-failed); workers carry it on their file_finished record, and the
fan-out parent adds timeout/crash/worker-exit for the TU a worker died on.
tuCoverage() turns the requested file list plus outcomes into a
ScanCoverage (requested TUs with no outcome are frontend-failed). SARIF
gets a run `invocations` entry (executionSuccessful, one
toolExecutionNotification per gap); findings JSON gets an additive
`coverage` object. A scan with any gap, or with nothing analyzed, exits 2
after writing its report. CLI diagnostics now go to stderr, as
docs/internal/cli-philosophy.md principle 12 requires. USAGE.md and
AGENTS.md document the coverage fields and the exit code.
Test: tests/unit/test_scan_coverage.cpp (classification, path spelling,
complete/untracked controls, merge, both writers, and unchanged output
without coverage); test_cli_paths "[coverage]" covers a clean TU (exit 0,
complete) and a TU with a missing include (exit 2, gap in JSON, SARIF on
stdout parseable with executionSuccessful=false). Before: both probe files
exited 0 with no coverage information. Full suite: 139/139.
