feat(resolver): apply config.platform overrides on top of detected platform

Mirrors `Composer\Repository\PlatformRepository`'s `$overrides` handling:
each override either replaces a detected platform package version or adds
a virtual one (e.g. ext-dummy), and `false` disables the package.

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

3 files changed, 33 insertions, 2 deletions

diff --git a/crates/mozart-registry/src/resolver.rs b/crates/mozart-registry/src/resolver.rs
index da2f444..6499130 100644
--- a/crates/mozart-registry/src/resolver.rs
+++ b/crates/mozart-registry/src/resolver.rs
@@ -295,6 +295,28 @@ impl PlatformConfig {
         Self { packages }
     }
 
+    /// Apply `config.platform` overrides on top of the detected packages.
+    ///
+    /// Mirrors `Composer\Repository\PlatformRepository::__construct`'s
+    /// `$overrides` handling: each override either replaces a detected
+    /// package version or adds a virtual one (e.g. `ext-dummy`). A `false`
+    /// value disables the package, removing it from the platform.
+    pub fn apply_overrides(&mut self, overrides: &serde_json::Value) {
+        let Some(obj) = overrides.as_object() else {
+            return;
+        };
+        for (name, value) in obj {
+            let key = name.to_lowercase();
+            if value.as_bool() == Some(false) {
+                self.packages.remove(&key);
+                continue;
+            }
+            if let Some(s) = value.as_str() {
+                self.packages.insert(key, s.to_string());
+            }
+        }
+    }
+
     /// Parse platform packages into `Version` values.
     pub fn to_versions(&self) -> HashMap<String, Version> {
         self.packages
diff --git a/crates/mozart/src/commands/update.rs b/crates/mozart/src/commands/update.rs
index 33b305a..17b7c97 100644
--- a/crates/mozart/src/commands/update.rs
+++ b/crates/mozart/src/commands/update.rs
@@ -872,6 +872,15 @@ pub async fn run(
         .unwrap_or(false);
     let prefer_stable = args.prefer_stable || composer_prefer_stable;
 
+    let mut platform = PlatformConfig::new();
+    if let Some(overrides) = composer_json
+        .extra_fields
+        .get("config")
+        .and_then(|c| c.get("platform"))
+    {
+        platform.apply_overrides(overrides);
+    }
+
     let request = ResolveRequest {
         root_name: composer_json.name.clone(),
         require,
@@ -881,7 +890,7 @@ pub async fn run(
         stability_flags: HashMap::new(),
         prefer_stable,
         prefer_lowest: args.prefer_lowest,
-        platform: PlatformConfig::new(),
+        platform,
         ignore_platform_reqs: args.ignore_platform_reqs,
         ignore_platform_req_list: args.ignore_platform_req.clone(),
         repositories: repositories.clone(),
diff --git a/crates/mozart/tests/installer.rs b/crates/mozart/tests/installer.rs
index a13e8e6..3daaf1e 100644
--- a/crates/mozart/tests/installer.rs
+++ b/crates/mozart/tests/installer.rs
@@ -273,7 +273,7 @@ installer_fixture!(install_ignore_platform_package_requirement_list);
 installer_fixture!(install_ignore_platform_package_requirement_wildcard);
 installer_fixture!(install_ignore_platform_package_requirements);
 installer_fixture!(install_missing_alias_from_lock, ignore);
-installer_fixture!(install_overridden_platform_packages, ignore);
+installer_fixture!(install_overridden_platform_packages);
 installer_fixture!(install_package_and_its_provider_skips_original);
 installer_fixture!(install_prefers_repos_over_package_versions);
 installer_fixture!(install_reference);