fix(update): apply --minimal-changes via policy preferred versions

The previous implementation pinned every resolved package back to its
locked version after the resolve, which discarded the new versions the
solver had to pick when a root constraint moved off the lock (e.g. a
require bumped from `1.*` to `2.*`). The lock effectively never moved,
so transitive cascades from a forced root-level update were lost.

Mirror Composer's `Installer::createPolicy(forUpdate=true,
minimalUpdate=true)` instead: thread the lock's `name → normalized
version` map through the policy as `preferred_versions`. The solver now
picks the locked version as a tiebreaker when it still satisfies the
active constraints, but moves freely when a constraint forces a
different version. Drop the post-process hook entirely.

1 files changed, 41 insertions, 0 deletions

diff --git a/crates/mozart-sat-resolver/src/policy.rs b/crates/mozart-sat-resolver/src/policy.rs
index a253c39..f45c4f5 100644
--- a/crates/mozart-sat-resolver/src/policy.rs
+++ b/crates/mozart-sat-resolver/src/policy.rs
@@ -10,6 +10,14 @@ pub struct DefaultPolicy {
     pub prefer_stable: bool,
     /// Whether to prefer lowest versions.
     pub prefer_lowest: bool,
+    /// `name → normalized version` overrides used when more than one
+    /// candidate could satisfy a requirement: a literal pinned at the
+    /// preferred version wins outright over the usual highest/lowest pick.
+    /// Mirrors Composer's `DefaultPolicy::pruneToBestVersion` behavior under
+    /// `--minimal-changes`, where the lock's previously-installed versions
+    /// are passed in so the solver only moves a package when a constraint
+    /// actually forces a different version.
+    pub preferred_versions: Option<IndexMap<String, String>>,
 }
 
 impl DefaultPolicy {
@@ -17,6 +25,19 @@ impl DefaultPolicy {
         DefaultPolicy {
             prefer_stable,
             prefer_lowest,
+            preferred_versions: None,
+        }
+    }
+
+    pub fn with_preferred(
+        prefer_stable: bool,
+        prefer_lowest: bool,
+        preferred_versions: IndexMap<String, String>,
+    ) -> Self {
+        DefaultPolicy {
+            prefer_stable,
+            prefer_lowest,
+            preferred_versions: Some(preferred_versions),
         }
     }
 
@@ -123,6 +144,26 @@ impl DefaultPolicy {
             return vec![];
         }
 
+        // Mirror Composer's `DefaultPolicy::pruneToBestVersion` short-circuit:
+        // when a preferred version is set for this package and one of the
+        // candidates matches it exactly, that wins over the regular
+        // highest/lowest pick. Falls through otherwise (e.g. the locked
+        // version no longer satisfies the constraint and was filtered out
+        // before reaching this method).
+        if let Some(ref preferred) = self.preferred_versions {
+            let name = pool.literal_to_package(literals[0]).name.clone();
+            if let Some(preferred_ver) = preferred.get(&name) {
+                let preferred_lits: Vec<Literal> = literals
+                    .iter()
+                    .filter(|&&lit| pool.literal_to_package(lit).version == *preferred_ver)
+                    .copied()
+                    .collect();
+                if !preferred_lits.is_empty() {
+                    return preferred_lits;
+                }
+            }
+        }
+
         // The first literal is the best after sorting
         let best_version = &pool.literal_to_package(literals[0]).version;
         literals