Fix bugs and performance issues in JSI interpreter

[Copilot]

Several correctness bugs and performance issues identified in the interpreter's runtime evaluation and object model.

Bug Fixes

• Object literal class type (context.rs): new_object was calling create_object with ClassType::Array instead of ClassType::Object, causing {} literals to be misidentified as arrays (breaking Object.prototype.toString etc.)

• Double expression evaluation (context.rs): In new_object, computed property names were evaluated twice — once into an unused variable x, then again for the actual name. Side-effecting expressions ran twice.

• Logical &&/|| return values (context.rs): Both operators were coercing their result to Value::Boolean instead of returning the actual operand. JS short-circuit semantics require returning the operand value:

  null || 'default'  // was: true  → now: 'default'
  obj && obj.prop    // was: false → now: null (when obj is null)
  1 && 2             // was: true  → now: 2

• String.prototype.charAt encoding (string.rs): u16::to_string() returns the decimal integer representation (e.g. "65" for 'A'), not the character. Fixed with char::from_u32(code as u32).

Performance Fixes

• close_scope (context.rs): Loop over child scopes continued after finding the match. Added break.

• Atomic ID counter (object.rs): Object::new and force_copy were doing fetch_add + load to get the new ID. fetch_add returns the previous value, so a single fetch_add(...) + 1 suffices.

Original prompt

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This section details on the original issue you should resolve

<issue_title>bug fix</issue_title>
<issue_description>检测当前的实现是否存在性能和实现的bug，帮忙在尽量少改动的情况下修复问题、提升性能</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes bug fix #30

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