feat: add map_layout and small and quiet repo modes - #92

• Add map_layout to .stacklitrc.json and STACKLIT_MAP_LAYOUT: flat keeps one line per module path, tree prints one line per parent directory
• Stop padding or truncating module paths at 30 characters, and drop a purpose that only repeats the directory name
• Add small_repo_files and quiet_repo_files thresholds to config and setup
• Cover the new config keys and derive output with tests

What and why

How it was checked

• go vet ./... and go test ./... -race pass
• Added or updated tests for behavior changes
• Ran the built binary on a real repo (name it):

Docs

• README, docs/USAGE.md or CHANGELOG.md updated if behavior changed
• Any size or speed claim comes from go run ./bench output
• Planned or unmerged features are not described as shipped