style: apply cargo fmt across the crate (A20)

The repo never enforced rustfmt, so formatting had drifted broadly. This is a
single mechanical `cargo fmt` pass over the whole crate (no behavioral change;
lib suite green, 493 passed). Going forward fmt should be enforced (planned CI
fmt --check step). Part of the 0.6.1 hygiene pass.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/identity.rs

@@ -41,7 +41,8 @@ pub fn identity_path() -> Option<PathBuf> {
 /// A *missing* file (first ever run, or right after a reset) is the normal
 /// create path.
 pub fn load_or_create() -> Result<SecretKey> {
-    let path = identity_path().context("could not determine a config directory for the identity key")?;
+    let path =
+        identity_path().context("could not determine a config directory for the identity key")?;
     load_or_create_at(&path)
 }
 
@@ -49,7 +50,8 @@ pub fn load_or_create() -> Result<SecretKey> {
 /// deliberate "Regenerate identity" / unlink action — the old id is discarded and
 /// unrecoverable, so callers should confirm with the user first.
 pub fn regenerate() -> Result<SecretKey> {
-    let path = identity_path().context("could not determine a config directory for the identity key")?;
+    let path =
+        identity_path().context("could not determine a config directory for the identity key")?;
     let key = SecretKey::generate();
     save_at(&path, &key)?;
     Ok(key)
@@ -57,7 +59,8 @@ pub fn regenerate() -> Result<SecretKey> {
 
 /// Atomic, `0600` write at the default identity path. See [`save_at`].
 pub fn save(key: &SecretKey) -> Result<()> {
-    let path = identity_path().context("could not determine a config directory for the identity key")?;
+    let path =
+        identity_path().context("could not determine a config directory for the identity key")?;
     save_at(&path, key)
 }
 
@@ -80,13 +83,15 @@ fn load_or_create_at(path: &std::path::Path) -> Result<SecretKey> {
 /// perms are applied before the rename so the secret is never briefly
 /// world-readable.
 fn save_at(path: &std::path::Path, key: &SecretKey) -> Result<()> {
-    let parent = path.parent().context("identity path has no parent directory")?;
+    let parent = path
+        .parent()
+        .context("identity path has no parent directory")?;
     fs::create_dir_all(parent).with_context(|| format!("failed to create {}", parent.display()))?;
 
     let tmp = parent.join(format!(".identity.key.tmp.{}", std::process::id()));
     {
-        let mut f =
-            fs::File::create(&tmp).with_context(|| format!("failed to create {}", tmp.display()))?;
+        let mut f = fs::File::create(&tmp)
+            .with_context(|| format!("failed to create {}", tmp.display()))?;
         #[cfg(unix)]
         {
             use std::os::unix::fs::PermissionsExt;