[pederbe]

Summary

Repeated fnm install calls succeed, but their already-installed warning was logged at error level. Log it at info level so it goes to stdout and is suppressed by --log-level error and quiet.

Add shell-matrix regression coverage for output streams, verbosity, successful exit status, and genuine errors. Update the existing warning test and add a patch changeset.

Fixes #1461.

Testing

• cargo test --locked: 28 passed.
• cargo fmt --all --check: passed.
• cargo clippy --locked --all-targets: passed with warnings in unchanged code.
• PowerShell issue-1461 regression: passed using the local Node mirror.
• Existing installation and log-level PowerShell tests: all assertions and selected snapshots passed; the filtered Jest invocation exits 1 because unselected shell snapshots are reported as obsolete.
• Prettier check on changed TypeScript and changeset: passed.
• tsc --noEmit: blocked by the existing TS5110 module/moduleResolution mismatch.