Fix placeholder expression assignment ownership

[BenJule]

Summary

• release existing string storage before copy or move assignment
• guard both copy and move self-assignment
• preserve move ownership transfer by marking the source expression empty

Problem

The move-assignment operator overwrote an existing string allocation without releasing it first. The adjacent copy-assignment operator also deleted its own string before reading from the same object during self-assignment.

Validation

• git diff --check
• one GPG-signed commit
• targeted native validation remains delegated to Jenkins

No local full build was performed.