0.18 round 1+2 integration fixes

Two clippy issues surfaced when round 1 polish + round 2 FrameSink
migrations both landed on libfreemkv main:

- src/halt.rs: clippy::new_without_default fires when a public new()
  exists without Default. The polish pass dropped the derive thinking
  it was redundant — clippy disagrees, so add a manual impl that
  forwards to new(). Doc-comment notes why both exist.

- src/disc/read_error.rs:372: pre-existing
  assert_eq!(.., true) trips clippy::bool_assert_comparison. Pre-0.18
  precommits passed because that lint sat outside the gate; the
  round-2 commits brought enough new clippy surface that it now
  shows up. Trivial cleanup: assert!(...) instead of assert_eq!.

Single contributor: MattJackson.

src/halt.rs

@@ -20,10 +20,11 @@ use std::sync::atomic::{AtomicBool, Ordering};
 /// no `reset()` by design — construct a fresh `Halt` for a fresh
 /// operation.
 ///
-/// Construct with [`Halt::new`]. We intentionally don't derive
-/// `Default` — `Halt::new()` is more discoverable, matches the
-/// stdlib `Mutex::new` / `Arc::new` convention, and keeps the
-/// uncancelled-by-construction invariant in one named place.
+/// Construct with [`Halt::new`] (or [`Halt::default`]). The `Default`
+/// impl forwards to `new()` — both produce a fresh, uncancelled token.
+/// The pair exists because clippy's `new_without_default` lint requires
+/// `Default` whenever a public `new()` is present, even when the two
+/// would do exactly the same thing.
 #[derive(Clone, Debug)]
 pub struct Halt(Arc<AtomicBool>);
 
@@ -44,6 +45,12 @@ impl Halt {
     }
 }
 
+impl Default for Halt {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -103,5 +110,4 @@ mod tests {
         handle.join().unwrap();
         assert!(h.is_cancelled());
     }
-
 }