Replace buffer-based renderers with explicit RenderResult returns and
use cargo-styled output macros for consistency.

Extract lint registry loading from cmd_explain into a shared
program-level resource initialized via setup.

Add a new `explain <LINT>` command that queries the embedded lint
registry (generated by `build.rs`) and displays detailed information
about a specifoic lint, including its name, title, summary, active-on
target, default setting, and author.

Enable the `structural_renderer` feature and add `serde`/`serde_json`
dependencies to support JSON output rendering for command results.