Relicense the project under MIT

[yifanxuaaa]

What changed

• Replace the Apache-2.0 license text with the MIT License
• Set the Rust workspace package license identifier to MIT
• Update the README license badge and statement

Why

The project is moving to the MIT License.

Validation

• git diff --check
• Confirmed every workspace package reports license = "MIT" via cargo metadata
• Confirmed no Apache license references remain