Add contract version query function

[big14way]

Expose the contract version for compatibility checking.

Acceptance Criteria:

• Add version() -> u32 public function
• Store version as constant
• Return version in a standard format
• Useful for SDK compatibility checks

Files: contracts/sorosave/src/lib.rs

[WuXieSec]

I can take this issue and submit an implementation PR.

Compliance note: I will only work on tasks that are legal and compliant.