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>

9 files changed, 34 insertions, 26 deletions

diff --git a/crates/mozart-vcs/Cargo.toml b/crates/mozart-vcs/Cargo.toml
index dc6dfc3..18eff25 100644
--- a/crates/mozart-vcs/Cargo.toml
+++ b/crates/mozart-vcs/Cargo.toml
@@ -8,6 +8,7 @@ mozart-core.workspace = true
 mozart-semver.workspace = true
 anyhow.workspace = true
 base64.workspace = true
+indexmap.workspace = true
 regex.workspace = true
 reqwest.workspace = true
 serde.workspace = true
diff --git a/crates/mozart-vcs/src/driver/bitbucket.rs b/crates/mozart-vcs/src/driver/bitbucket.rs
index d47987d..77704fa 100644
--- a/crates/mozart-vcs/src/driver/bitbucket.rs
+++ b/crates/mozart-vcs/src/driver/bitbucket.rs
@@ -1,4 +1,5 @@
-use std::collections::{BTreeMap, HashMap};
+use indexmap::IndexMap;
+use std::collections::BTreeMap;
 
 use anyhow::{Result, bail};
 use regex::Regex;
@@ -16,7 +17,7 @@ pub struct BitbucketDriver {
     root_identifier: Option<String>,
     tags: Option<BTreeMap<String, String>>,
     branches: Option<BTreeMap<String, String>>,
-    info_cache: HashMap<String, Option<serde_json::Value>>,
+    info_cache: IndexMap<String, Option<serde_json::Value>>,
     git_driver: Option<Box<GitDriver>>,
     http_client: Client,
     config: DriverConfig,
@@ -34,7 +35,7 @@ impl BitbucketDriver {
             root_identifier: None,
             tags: None,
             branches: None,
-            info_cache: HashMap::new(),
+            info_cache: IndexMap::new(),
             git_driver: None,
             http_client: Client::new(),
             config,
diff --git a/crates/mozart-vcs/src/driver/forgejo.rs b/crates/mozart-vcs/src/driver/forgejo.rs
index ec2ca14..488e165 100644
--- a/crates/mozart-vcs/src/driver/forgejo.rs
+++ b/crates/mozart-vcs/src/driver/forgejo.rs
@@ -1,4 +1,5 @@
-use std::collections::{BTreeMap, HashMap};
+use indexmap::IndexMap;
+use std::collections::BTreeMap;
 
 use anyhow::{Result, bail};
 use regex::Regex;
@@ -20,7 +21,7 @@ pub struct ForgejoDriver {
     root_identifier: Option<String>,
     tags: Option<BTreeMap<String, String>>,
     branches: Option<BTreeMap<String, String>>,
-    info_cache: HashMap<String, Option<serde_json::Value>>,
+    info_cache: IndexMap<String, Option<serde_json::Value>>,
     git_driver: Option<Box<GitDriver>>,
     http_client: Client,
     config: DriverConfig,
@@ -39,7 +40,7 @@ impl ForgejoDriver {
             root_identifier: None,
             tags: None,
             branches: None,
-            info_cache: HashMap::new(),
+            info_cache: IndexMap::new(),
             git_driver: None,
             http_client: Client::new(),
             config,
diff --git a/crates/mozart-vcs/src/driver/git.rs b/crates/mozart-vcs/src/driver/git.rs
index cc9a210..43f4ecb 100644
--- a/crates/mozart-vcs/src/driver/git.rs
+++ b/crates/mozart-vcs/src/driver/git.rs
@@ -1,4 +1,5 @@
-use std::collections::{BTreeMap, HashMap};
+use indexmap::IndexMap;
+use std::collections::BTreeMap;
 use std::path::{Path, PathBuf};
 
 use anyhow::Result;
@@ -17,7 +18,7 @@ pub struct GitDriver {
     root_identifier: Option<String>,
     tags: Option<BTreeMap<String, String>>,
     branches: Option<BTreeMap<String, String>>,
-    info_cache: HashMap<String, Option<serde_json::Value>>,
+    info_cache: IndexMap<String, Option<serde_json::Value>>,
     git_util: GitUtil,
     is_local: bool,
 }
@@ -37,7 +38,7 @@ impl GitDriver {
             root_identifier: None,
             tags: None,
             branches: None,
-            info_cache: HashMap::new(),
+            info_cache: IndexMap::new(),
             git_util,
             is_local,
         }
@@ -85,7 +86,7 @@ impl GitDriver {
     fn parse_tags(output: &str) -> BTreeMap<String, String> {
         let mut tags = BTreeMap::new();
         // First pass: collect dereferenced tags (^{})
-        let mut dereferenced = HashMap::new();
+        let mut dereferenced = IndexMap::new();
         for line in output.lines() {
             let line = line.trim();
             if line.is_empty() {
diff --git a/crates/mozart-vcs/src/driver/github.rs b/crates/mozart-vcs/src/driver/github.rs
index c47c2fe..9c11389 100644
--- a/crates/mozart-vcs/src/driver/github.rs
+++ b/crates/mozart-vcs/src/driver/github.rs
@@ -1,4 +1,5 @@
-use std::collections::{BTreeMap, HashMap};
+use indexmap::IndexMap;
+use std::collections::BTreeMap;
 
 use anyhow::{Result, bail};
 use regex::Regex;
@@ -19,7 +20,7 @@ pub struct GitHubDriver {
     tags: Option<BTreeMap<String, String>>,
     branches: Option<BTreeMap<String, String>>,
     repo_data: Option<serde_json::Value>,
-    info_cache: HashMap<String, Option<serde_json::Value>>,
+    info_cache: IndexMap<String, Option<serde_json::Value>>,
     git_driver: Option<Box<GitDriver>>,
     http_client: Client,
     config: DriverConfig,
@@ -37,7 +38,7 @@ impl GitHubDriver {
             tags: None,
             branches: None,
             repo_data: None,
-            info_cache: HashMap::new(),
+            info_cache: IndexMap::new(),
             git_driver: None,
             http_client: Client::new(),
             config,
diff --git a/crates/mozart-vcs/src/driver/gitlab.rs b/crates/mozart-vcs/src/driver/gitlab.rs
index f96c078..c1afbcb 100644
--- a/crates/mozart-vcs/src/driver/gitlab.rs
+++ b/crates/mozart-vcs/src/driver/gitlab.rs
@@ -1,4 +1,5 @@
-use std::collections::{BTreeMap, HashMap};
+use indexmap::IndexMap;
+use std::collections::BTreeMap;
 
 use anyhow::{Result, bail};
 use regex::Regex;
@@ -21,7 +22,7 @@ pub struct GitLabDriver {
     root_identifier: Option<String>,
     tags: Option<BTreeMap<String, String>>,
     branches: Option<BTreeMap<String, String>>,
-    info_cache: HashMap<String, Option<serde_json::Value>>,
+    info_cache: IndexMap<String, Option<serde_json::Value>>,
     git_driver: Option<Box<GitDriver>>,
     http_client: Client,
     config: DriverConfig,
@@ -41,7 +42,7 @@ impl GitLabDriver {
             root_identifier: None,
             tags: None,
             branches: None,
-            info_cache: HashMap::new(),
+            info_cache: IndexMap::new(),
             git_driver: None,
             http_client: Client::new(),
             config,
diff --git a/crates/mozart-vcs/src/driver/hg.rs b/crates/mozart-vcs/src/driver/hg.rs
index f884c50..0782775 100644
--- a/crates/mozart-vcs/src/driver/hg.rs
+++ b/crates/mozart-vcs/src/driver/hg.rs
@@ -1,4 +1,5 @@
-use std::collections::{BTreeMap, HashMap};
+use indexmap::IndexMap;
+use std::collections::BTreeMap;
 use std::path::PathBuf;
 
 use anyhow::Result;
@@ -17,7 +18,7 @@ pub struct HgDriver {
     root_identifier: Option<String>,
     tags: Option<BTreeMap<String, String>>,
     branches: Option<BTreeMap<String, String>>,
-    info_cache: HashMap<String, Option<serde_json::Value>>,
+    info_cache: IndexMap<String, Option<serde_json::Value>>,
     hg_util: HgUtil,
     config: DriverConfig,
 }
@@ -31,7 +32,7 @@ impl HgDriver {
             root_identifier: None,
             tags: None,
             branches: None,
-            info_cache: HashMap::new(),
+            info_cache: IndexMap::new(),
             hg_util: HgUtil::new(process),
             config,
         }
diff --git a/crates/mozart-vcs/src/driver/svn.rs b/crates/mozart-vcs/src/driver/svn.rs
index eea2d08..16363e1 100644
--- a/crates/mozart-vcs/src/driver/svn.rs
+++ b/crates/mozart-vcs/src/driver/svn.rs
@@ -1,4 +1,5 @@
-use std::collections::{BTreeMap, HashMap};
+use indexmap::IndexMap;
+use std::collections::BTreeMap;
 
 use anyhow::Result;
 use regex::Regex;
@@ -20,7 +21,7 @@ pub struct SvnDriver {
     root_identifier: Option<String>,
     tags: Option<BTreeMap<String, String>>,
     branches: Option<BTreeMap<String, String>>,
-    info_cache: HashMap<String, Option<serde_json::Value>>,
+    info_cache: IndexMap<String, Option<serde_json::Value>>,
     svn_util: SvnUtil,
 }
 
@@ -36,7 +37,7 @@ impl SvnDriver {
             root_identifier: None,
             tags: None,
             branches: None,
-            info_cache: HashMap::new(),
+            info_cache: IndexMap::new(),
             svn_util: SvnUtil::new(process),
         }
     }
diff --git a/crates/mozart-vcs/src/process.rs b/crates/mozart-vcs/src/process.rs
index 91741a8..8ccc11d 100644
--- a/crates/mozart-vcs/src/process.rs
+++ b/crates/mozart-vcs/src/process.rs
@@ -1,4 +1,4 @@
-use std::collections::HashMap;
+use indexmap::IndexMap;
 use std::path::Path;
 use std::process::Command;
 use std::time::{Duration, Instant};
@@ -18,7 +18,7 @@ pub struct ProcessOutput {
 /// Corresponds to Composer's `ProcessExecutor`.
 pub struct ProcessExecutor {
     timeout: Option<Duration>,
-    env_overrides: HashMap<String, Option<String>>,
+    env_overrides: IndexMap<String, Option<String>>,
 }
 
 impl Default for ProcessExecutor {
@@ -31,14 +31,14 @@ impl ProcessExecutor {
     pub fn new() -> Self {
         Self {
             timeout: None,
-            env_overrides: HashMap::new(),
+            env_overrides: IndexMap::new(),
         }
     }
 
     pub fn with_timeout(secs: u64) -> Self {
         Self {
             timeout: Some(Duration::from_secs(secs)),
-            env_overrides: HashMap::new(),
+            env_overrides: IndexMap::new(),
         }
     }