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>

2 files changed, 5 insertions, 4 deletions

diff --git a/crates/mozart-test-harness/Cargo.toml b/crates/mozart-test-harness/Cargo.toml
index 61b9109..6c9421c 100644
--- a/crates/mozart-test-harness/Cargo.toml
+++ b/crates/mozart-test-harness/Cargo.toml
@@ -5,6 +5,7 @@ edition.workspace = true
 
 [dependencies]
 anyhow.workspace = true
+indexmap.workspace = true
 regex.workspace = true
 serde_json.workspace = true
 tempfile.workspace = true
diff --git a/crates/mozart-test-harness/src/parser.rs b/crates/mozart-test-harness/src/parser.rs
index dbc71ae..827272b 100644
--- a/crates/mozart-test-harness/src/parser.rs
+++ b/crates/mozart-test-harness/src/parser.rs
@@ -1,5 +1,5 @@
 use anyhow::{Context, Result, bail};
-use std::collections::HashMap;
+use indexmap::IndexMap;
 use std::fs;
 use std::path::Path;
 
@@ -53,7 +53,7 @@ pub fn parse_test_str(content: &str) -> Result<ParsedTest> {
         }
     }
 
-    let mut take = |key: &str| sections.remove(key);
+    let mut take = |key: &str| sections.shift_remove(key);
 
     let test = take("TEST").unwrap();
     let composer = take("COMPOSER").unwrap();
@@ -86,10 +86,10 @@ pub fn parse_test_str(content: &str) -> Result<ParsedTest> {
     })
 }
 
-fn split_sections(content: &str) -> Result<HashMap<String, String>> {
+fn split_sections(content: &str) -> Result<IndexMap<String, String>> {
     let header_re = regex::Regex::new(r"^--([A-Z][A-Z-]*)--$").unwrap();
 
-    let mut sections: HashMap<String, String> = HashMap::new();
+    let mut sections: IndexMap<String, String> = IndexMap::new();
     let mut current_section: Option<String> = None;
     let mut current_body = String::new();