Add consume-and-forget protected expression compilation

[imakris]

Summary

• add set_protected_expression() for x86-64 SSE2 compilation of sensitive formulas
• scrub MEXCE-owned source, token, temporary code, and compiler-state buffers
• disable IR and machine-code introspection for protected expressions
• invalidate stale compiled state on compilation failure and safely handle unbinding
• document the security boundary and limitations

Validation

• MSVC x64 and x86 unit suites pass locally
• this PR intentionally exercises the repository's Ubuntu and Windows CI matrix to verify the protected path on Linux x86-64

Protected compilation reduces plaintext residency; it does not encrypt generated machine code or prevent same-process debugging/disassembly.