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.

6 files changed, 139 insertions, 4 deletions

diff --git a/mingling_cli/Cargo.lock b/mingling_cli/Cargo.lock
index 5a0dcd0..60f8792 100644
--- a/mingling_cli/Cargo.lock
+++ b/mingling_cli/Cargo.lock
@@ -277,6 +277,7 @@ dependencies = [
  "arg-picker",
  "mingling_core",
  "mingling_macros",
+ "serde",
 ]
 
 [[package]]
@@ -304,6 +305,8 @@ dependencies = [
  "just_template",
  "might_be_async",
  "mingling_pathf",
+ "serde",
+ "serde_json",
 ]
 
 [[package]]
diff --git a/mingling_cli/Cargo.toml b/mingling_cli/Cargo.toml
index 4dfea20..a7a8538 100644
--- a/mingling_cli/Cargo.toml
+++ b/mingling_cli/Cargo.toml
@@ -27,7 +27,8 @@ features = [
     "pathf",
     "dispatch_tree",
     "async",
-    "comp"
+    "comp",
+    "structural_renderer"
 ]
 
 [build-dependencies.mingling]
diff --git a/mingling_cli/help/help.txt b/mingling_cli/help/help.txt
index d5073e6..2089e2b 100644
--- a/mingling_cli/help/help.txt
+++ b/mingling_cli/help/help.txt
@@ -17,7 +17,7 @@ COMMANDS:
    ra-lint                        Run `mling lint` and output the results
    ra-lint-check                  Run `mling lint` and `cargo check`, and output the combined results
    ra-lint-clippy                 Run `mling lint` and `cargo clippy`, and output the combined results
-   explain <LINT>                 [NOT_IMPL] Explain the meaning of the specified Lint
+   explain <LINT>                 Explain the meaning of the specified Lint
 
   PROJECT MANAGE:
    proj-new <NAME>                [NOT_IMPL] Create a project
diff --git a/mingling_cli/src/linter.rs b/mingling_cli/src/linter.rs
index 45111b6..4916eba 100644
--- a/mingling_cli/src/linter.rs
+++ b/mingling_cli/src/linter.rs
@@ -1,4 +1,6 @@
+pub mod cmd_explain;
 pub mod cmd_lint;
 pub mod cmd_ra_lints;
 pub mod mlint_attr;
 pub mod mlint_report;
+
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);
+    }
+}
diff --git a/mingling_cli/src/main.rs b/mingling_cli/src/main.rs
index cb41368..da30861 100644
--- a/mingling_cli/src/main.rs
+++ b/mingling_cli/src/main.rs
@@ -7,9 +7,12 @@ use crate::metadata::{
 };
 use mingling::{
     ShellContext, Suggest,
-    consts::HELP_FLAG,
+    consts::{HELP_FLAG, JSON_FLAG, JSON_PRETTY_FLAG},
     macros::{completion, gen_program, help, suggest},
-    setup::{ExitCodeSetup, picker::HelpFlagSetup},
+    setup::{
+        ExitCodeSetup,
+        picker::{HelpFlagSetup, StructuralRendererSetup},
+    },
 };
 
 pub mod diagnostic;
@@ -26,6 +29,7 @@ async fn main() {
     let mut program = ThisProgram::new();
 
     // Setups
+    program.with_setup(StructuralRendererSetup);
     program.with_setup(HelpFlagSetup::default());
     program.with_setup(ExitCodeSetup::default());
 
@@ -50,6 +54,8 @@ pub fn complete_global(_ctx: &ShellContext) -> Suggest {
         ARG_ALL_FEATURES: "Enable all features",
         ARG_NO_DEFAULT_FEATURES: "Disable default features",
         ARG_NO_DEPS: "Do not include dependencies in metadata",
+        JSON_FLAG: "Render results in JSON format",
+        JSON_PRETTY_FLAG: "Render results in pretty JSON format"
     }
 }