refactor: switch internal maps/sets from HashMap to IndexMap

Adopt indexmap workspace-wide so iteration order is deterministic and
follows insertion order. The non-deterministic order of std HashMap
otherwise leaks into resolver decisions when multiple valid solutions
exist (e.g. cyclic require pairs under prefer-lowest), making behavior
flaky and divergent from Composer's PHP-array semantics.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

1 files changed, 3 insertions, 3 deletions

diff --git a/crates/mozart-sat-resolver/src/rule_watch_graph.rs b/crates/mozart-sat-resolver/src/rule_watch_graph.rs
index 1b7604d..202dcca 100644
--- a/crates/mozart-sat-resolver/src/rule_watch_graph.rs
+++ b/crates/mozart-sat-resolver/src/rule_watch_graph.rs
@@ -2,7 +2,7 @@ use crate::decisions::Decisions;
 use crate::pool::Literal;
 use crate::rule::Rule;
 use crate::rule_set::RuleId;
-use std::collections::HashMap;
+use indexmap::IndexMap;
 
 /// A watch node: tracks which 2 literals a rule watches.
 ///
@@ -24,7 +24,7 @@ struct WatchNode {
 /// Port of Composer's RuleWatchGraph.php.
 pub struct RuleWatchGraph {
     /// Literal → list of watch node indices watching that literal.
-    watch_chains: HashMap<Literal, Vec<usize>>,
+    watch_chains: IndexMap<Literal, Vec<usize>>,
     /// All watch nodes.
     nodes: Vec<WatchNode>,
 }
@@ -32,7 +32,7 @@ pub struct RuleWatchGraph {
 impl RuleWatchGraph {
     pub fn new() -> Self {
         RuleWatchGraph {
-            watch_chains: HashMap::new(),
+            watch_chains: IndexMap::new(),
             nodes: Vec::new(),
         }
     }