CLI output (experimental)

The structured output implementation is being developed incrementally. Currently these global options apply to diagnostics emitted through the logger:

dcr --output plain build
dcr --output json build
dcr --color never build
dcr --quiet build
dcr --verbose build

--output accepts human (default), plain, or json. Diagnostics are written to stderr. JSON diagnostics are newline-delimited objects with schema_version: 1, event, and message fields. Command statuses use the shared output helpers, build reporting emits structured lifecycle events, and compiler output is represented as JSON events with its original stream and text. The protocol is still experimental: some raw command data and subprocess stdout remain unstructured.

--color accepts auto, always, or never. Automatic coloring requires a terminal and respects NO_COLOR. Plain and JSON diagnostics never add colors. Quiet mode preserves warnings, errors, and compiler diagnostics; verbose mode enables debug and trace logging. Arguments following -- are passed through unchanged.

Remaining work includes migrating the remaining raw command data and subprocess stdout, improving progress rendering and contextual errors, and expanding integration coverage for stdout/stderr and closed pipes.