feat(cli): implement `explain` command and add JSON structural rendering

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.

1 files changed, 123 insertions, 0 deletions

diff --git a/mingling_cli/src/linter/cmd_explain.rs b/mingling_cli/src/linter/cmd_explain.rs
new file mode 100644
index 0000000..d76cf65
--- /dev/null
+++ b/mingling_cli/src/linter/cmd_explain.rs
@@ -0,0 +1,123 @@
+use mingling::{
+    Grouped, Routable, StructuralData,
+    macros::{
+        arg, buffer, chain, dispatcher, metadata, pack, pack_err_structural, r_println, renderer,
+        routeify,
+    },
+    metadata::Description,
+    picker::EntryPicker,
+};
+use serde::{Deserialize, Serialize};
+use std::sync::OnceLock;
+
+use crate::Next;
+
+dispatcher!("explain");
+
+#[metadata(EntryExplain)]
+pub fn desc_explain() -> Description {
+    "Explain the meaning of the specified Lint".into()
+}
+
+pack!(StateExplainLint = String);
+pack_err_structural!(ErrorNoExplainLintProvided);
+pack_err_structural!(ErrorNoSuchLint = String);
+
+#[derive(Debug, Default, Grouped, StructuralData, Serialize)]
+pub struct ResultExplainLint {
+    pub lint_name: String,
+    pub title: String,
+    pub summary: String,
+    pub active_on: String,
+    pub author: String,
+    pub default: String,
+}
+
+#[chain(routeify)]
+pub fn handle_explain(args: EntryExplain) -> Next {
+    let lint_name = args
+        .pick_or_route(&arg![String], || {
+            ErrorNoExplainLintProvided::default().to_chain()
+        })
+        .to_result()?;
+    StateExplainLint::new(lint_name).into()
+}
+
+/// Mirror of the lint registry JSON generated by `build.rs` (`registry.json`).
+#[derive(Debug, Deserialize)]
+struct LintRegistry {
+    lints: Vec<LintEntry>,
+}
+
+#[derive(Debug, Deserialize)]
+struct LintEntry {
+    name: String,
+    title: String,
+    summary: String,
+    metadata: LintMetadata,
+}
+
+#[derive(Debug, Deserialize)]
+struct LintMetadata {
+    active_on: String,
+    author: String,
+    default: String,
+}
+
+/// The lint registry, embedded at compile time via `include_str!`.
+///
+/// `registry.json` is regenerated by `build.rs` on every build, so the
+/// embedded copy always reflects the lints in `src/lints/`.
+fn lint_registry() -> &'static LintRegistry {
+    static REGISTRY: OnceLock<LintRegistry> = OnceLock::new();
+    REGISTRY.get_or_init(|| {
+        serde_json::from_str(include_str!("../../registry.json"))
+            .expect("failed to parse embedded registry.json")
+    })
+}
+
+#[chain]
+pub fn handle_state_explain_lint(p: StateExplainLint) -> Next {
+    let lint_name = p.inner;
+    let Some(entry) = lint_registry().lints.iter().find(|l| l.name == lint_name) else {
+        return ErrorNoSuchLint::new(lint_name).to_chain();
+    };
+    ResultExplainLint {
+        lint_name: entry.name.clone(),
+        title: entry.title.clone(),
+        summary: entry.summary.clone(),
+        active_on: entry.metadata.active_on.clone(),
+        author: entry.metadata.author.clone(),
+        default: entry.metadata.default.clone(),
+    }
+    .to_chain()
+}
+
+#[renderer(buffer)]
+pub fn render_explain_lint(r: ResultExplainLint) {
+    r_println!("{}", r.title);
+    r_println!("");
+    r_println!("  Name:      {}", r.lint_name);
+    r_println!("  Active on: {}", r.active_on);
+    r_println!("  Default:   {}", r.default);
+    r_println!("  Author:    {}", r.author);
+    r_println!("");
+    r_println!("{}", r.summary);
+}
+
+#[renderer(buffer)]
+pub fn render_error_no_explain_lint_provided(_: ErrorNoExplainLintProvided) {
+    r_println!("No lint name provided");
+    r_println!("");
+    r_println!("Usage: mling explain <LINT>");
+}
+
+#[renderer(buffer)]
+pub fn render_error_no_such_lint(err: ErrorNoSuchLint) {
+    r_println!("No such lint: \"{}\"", err.info);
+    r_println!("");
+    r_println!("Available lints:");
+    for entry in &lint_registry().lints {
+        r_println!("  {}", entry.name);
+    }
+}