fix(json): encode empty stdClass fields as {} not []

Empty PhpMixed::Array now serializes as [], so the several places that build a
PHP `new \stdClass` via an empty Array were emitting [] where Composer writes
{}. Use PhpMixed::Object for those empty-object cases:

- InitCommand: require / require-dev
- Locker::fixupJsonDataType: stability-flags / platform / platform-dev
- JsonConfigSource fallback: require/config keys that must stay objects

Align the affected test expectations with Composer: JsonFile::read() decodes
with assoc=true, collapsing the on-disk {} back to [], so reads expect [].

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

1 files changed, 4 insertions, 4 deletions

diff --git a/crates/shirabe/src/command/init_command.rs b/crates/shirabe/src/command/init_command.rs
index ab0e52c..88a2934 100644
--- a/crates/shirabe/src/command/init_command.rs
+++ b/crates/shirabe/src/command/init_command.rs
@@ -226,8 +226,8 @@ impl Command for InitCommand {
                 .unwrap_or_default();
             let formatted = self.format_requirements(req_list)?;
             if formatted.is_empty() {
-                // PHP: new \stdClass — represented as an empty IndexMap (JSON object)
-                PhpMixed::Array(IndexMap::new())
+                // PHP: new \stdClass — empty JSON object
+                PhpMixed::Object(IndexMap::new())
             } else {
                 PhpMixed::Array(
                     formatted
@@ -238,7 +238,7 @@ impl Command for InitCommand {
             }
         } else {
             // PHP: new \stdClass
-            PhpMixed::Array(IndexMap::new())
+            PhpMixed::Object(IndexMap::new())
         };
         options.insert("require".to_string(), require_value);
 
@@ -254,7 +254,7 @@ impl Command for InitCommand {
                 .unwrap_or_default();
             let formatted = self.format_requirements(req_list)?;
             let value = if formatted.is_empty() {
-                PhpMixed::Array(IndexMap::new())
+                PhpMixed::Object(IndexMap::new())
             } else {
                 PhpMixed::Array(
                     formatted