fix(lance-graph-contract): clippy::derivable_impls — derive Default with #[default] variant

CI `clippy` job (PR #404) flagged two `clippy::derivable_impls` warnings
in the 0.2.0 additive submodules. Both manual `impl Default` blocks
replaced with `#[derive(Default)]` on the enum + `#[default]` attribute
on the chosen variant:

* SupervisionPolicy: derive Default; mark OneForOne with #[default].
  Kept a const fn one_for_one() constructor for call-site readability.
* EngineHint: derive Default; mark Auto with #[default].

441/441 lance-graph-contract lib tests still pass. clippy clean.

CI signal: PR #404 clippy.

Author: claude

crates/lance-graph-contract/src/actor.rs

@@ -30,19 +30,22 @@ use core::time::Duration;
 /// Mirrors Erlang/OTP's classic three supervisor strategies. The
 /// `cognitive-shader-actor` crate maps these onto ractor's supervisor
 /// hierarchy; this enum is the contract vocabulary.
-#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
+#[derive(Clone, Copy, Debug, Default, PartialEq, Eq, Hash)]
 #[non_exhaustive]
 pub enum SupervisionPolicy {
     /// Restart only the failed child.
+    #[default]
     OneForOne,
     /// Restart all children on any failure (use when children share state).
     OneForAll,
     /// Restart the failed child and every child started after it.
     RestForOne,
 }
 
-impl Default for SupervisionPolicy {
-    fn default() -> Self {
+impl SupervisionPolicy {
+    /// Convenience constructor for the default policy. Equivalent to
+    /// `SupervisionPolicy::default()`, kept for call-site readability.
+    pub const fn one_for_one() -> Self {
         Self::OneForOne
     }
 }

crates/lance-graph-contract/src/ir.rs

@@ -76,7 +76,7 @@ impl fmt::Display for Cardinality {
 /// Adding a new engine is an additive enum variant — existing consumers
 /// continue to compile (they will get a `non_exhaustive`-style warning
 /// if they `match` exhaustively; mitigation: use `_ =>`).
-#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
+#[derive(Clone, Copy, Debug, Default, PartialEq, Eq, Hash)]
 #[non_exhaustive]
 pub enum EngineHint {
     /// Embedded LSM KV (RocksDB / SurrealKV).
@@ -92,15 +92,10 @@ pub enum EngineHint {
     /// Cognitive shader (lance-graph-cognitive crates).
     Cognitive,
     /// Engine selection is deferred to the planner's cost model.
+    #[default]
     Auto,
 }
 
-impl Default for EngineHint {
-    fn default() -> Self {
-        Self::Auto
-    }
-}
-
 /// Operator kind. The IR is intentionally coarse — it carries enough
 /// signal for routing decisions, not enough to be a full physical plan.
 ///