style: format the whole tree with cargo fmt (stock defaults, #35)

One-time, mechanical reformat — no functional changes. The tree was not
rustfmt-clean (~1800 hunks across ~100 files); this brings it to stock
`cargo fmt` defaults so a `cargo fmt --check` CI gate can follow.
Behaviour-preserving: 2509 pass / 0 fail / 1 ignored (unchanged baseline),
clippy clean. A .git-blame-ignore-revs entry follows so `git blame`
skips this commit.

src/cli.rs

@@ -96,10 +96,7 @@ pub enum ArgsError {
 impl std::fmt::Display for ArgsError {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         match self {
-            Self::MissingValue(flag) => f.write_str(&crate::t!(
-                "cli.missing_value",
-                flag = flag,
-            )),
+            Self::MissingValue(flag) => f.write_str(&crate::t!("cli.missing_value", flag = flag,)),
             Self::InvalidValue {
                 flag,
                 value,
@@ -110,10 +107,7 @@ impl std::fmt::Display for ArgsError {
                 value = value,
                 expected = expected,
             )),
-            Self::Unknown(arg) => f.write_str(&crate::t!(
-                "cli.unknown_argument",
-                arg = arg,
-            )),
+            Self::Unknown(arg) => f.write_str(&crate::t!("cli.unknown_argument", arg = arg,)),
             Self::MultiplePaths { first, second } => f.write_str(&crate::t!(
                 "cli.multiple_paths",
                 first = first,
@@ -261,7 +255,11 @@ fn default_theme() -> Theme {
         // Standard convention: 0..=6 and 8 are dark backgrounds,
         // 7 and 9..=15 are light. ITerm emits 15 for white-ish.
         let is_dark = matches!(code, 0..=6 | 8);
-        return if is_dark { Theme::dark() } else { Theme::light() };
+        return if is_dark {
+            Theme::dark()
+        } else {
+            Theme::light()
+        };
     }
     Theme::default()
 }
@@ -314,10 +312,19 @@ mod tests {
 
     #[test]
     fn mode_flag_simple_and_advanced() {
-        assert_eq!(Args::parse(["--mode", "simple"]).unwrap().mode, Some(Mode::Simple));
-        assert_eq!(Args::parse(["--mode", "advanced"]).unwrap().mode, Some(Mode::Advanced));
+        assert_eq!(
+            Args::parse(["--mode", "simple"]).unwrap().mode,
+            Some(Mode::Simple)
+        );
+        assert_eq!(
+            Args::parse(["--mode", "advanced"]).unwrap().mode,
+            Some(Mode::Advanced)
+        );
         // Case-insensitive, like the `mode` command.
-        assert_eq!(Args::parse(["--mode", "ADVANCED"]).unwrap().mode, Some(Mode::Advanced));
+        assert_eq!(
+            Args::parse(["--mode", "ADVANCED"]).unwrap().mode,
+            Some(Mode::Advanced)
+        );
     }
 
     #[test]
@@ -350,7 +357,10 @@ mod tests {
     #[test]
     fn data_dir_flag_parses() {
         let args = Args::parse(["--data-dir", "/tmp/playground-data"]).unwrap();
-        assert_eq!(args.data_dir.as_deref(), Some(std::path::Path::new("/tmp/playground-data")));
+        assert_eq!(
+            args.data_dir.as_deref(),
+            Some(std::path::Path::new("/tmp/playground-data"))
+        );
     }
 
     #[test]
@@ -370,13 +380,11 @@ mod tests {
 
     #[test]
     fn data_dir_and_positional_can_coexist() {
-        let args = Args::parse([
-            "--data-dir",
-            "/tmp/data",
-            "/home/me/MyProject",
-        ])
-        .unwrap();
-        assert_eq!(args.data_dir.as_deref(), Some(std::path::Path::new("/tmp/data")));
+        let args = Args::parse(["--data-dir", "/tmp/data", "/home/me/MyProject"]).unwrap();
+        assert_eq!(
+            args.data_dir.as_deref(),
+            Some(std::path::Path::new("/tmp/data"))
+        );
         assert_eq!(
             args.project_path.as_deref(),
             Some(std::path::Path::new("/home/me/MyProject"))
@@ -386,7 +394,10 @@ mod tests {
     #[test]
     fn two_positional_paths_error() {
         let err = Args::parse(["/a", "/b"]).unwrap_err();
-        assert!(matches!(err, ArgsError::MultiplePaths { .. }), "got: {err:?}");
+        assert!(
+            matches!(err, ArgsError::MultiplePaths { .. }),
+            "got: {err:?}"
+        );
     }
 
     #[test]
@@ -455,7 +466,10 @@ mod tests {
         // Absent `--demo` (and absent env var in the test runner),
         // demo mode is off — zero footprint for real users.
         let args = Args::parse(std::iter::empty::<&str>()).unwrap();
-        assert!(!args.demo, "demo is off unless --demo or the env var is given");
+        assert!(
+            !args.demo,
+            "demo is off unless --demo or the env var is given"
+        );
     }
 
     #[test]
@@ -484,7 +498,10 @@ mod tests {
         }
         // Disabling values.
         for v in ["", "  ", "0", "false", "False", "no", "off", "OFF"] {
-            assert!(!demo_value_is_truthy(v), "{v:?} should not enable demo mode");
+            assert!(
+                !demo_value_is_truthy(v),
+                "{v:?} should not enable demo mode"
+            );
         }
     }
 
@@ -493,7 +510,10 @@ mod tests {
         // Make sure the path-vs-flag distinction is robust:
         // unknown flags don't get silently swallowed as paths.
         let err = Args::parse(["--bogus", "/some/path"]).unwrap_err();
-        assert!(matches!(&err, ArgsError::Unknown(s) if s == "--bogus"), "got: {err:?}");
+        assert!(
+            matches!(&err, ArgsError::Unknown(s) if s == "--bogus"),
+            "got: {err:?}"
+        );
     }
 
     // ---- ADR-0054: --version / -V ----