Add consume-and-forget protected expression compilation

README.md

@@ -93,8 +93,35 @@ Compiles an expression, making it the default for `evaluate()`.
     *   Throws `mexce_parsing_exception` on syntax errors, providing the position of the error.
     *   Throws `std::logic_error` if the expression string is empty.
 
+#### `set_protected_expression()`
+Compiles sensitive source supplied by an authenticated formula producer.
+*   **Signature:** `void set_protected_expression(const std::string& expression);`
+*   **Behavior:**
+    *   Compiles through the x64 SSE2 backend using a scrubbed working buffer and discards live reconstructable compiler state.
+    *   Does not retain the caller-owned source; the caller remains responsible for its lifetime and erasure.
+    *   Rejects x87 and CSE options and disables expression and byte-code introspection for the compiled formula.
+    *   Allocator remnants and generated machine code can still reveal the formula to a debugger; this API reduces plaintext residency but does not prevent extraction by code running in the same process.
+
+```cpp
+double x = 2.0;
+mexce::evaluator eval;
+eval.bind(x, "x");
+
+std::string decrypted_source = "sin(x) + x * x";
+eval.set_protected_expression(decrypted_source);
+securely_erase_application_buffer(
+    decrypted_source); // Application-supplied secure erasure.
+
+const double result = eval.evaluate();
+```
+
+`set_protected_expression()` erases MEXCE's working copy. The application must
+erase `decrypted_source` with its own non-optimizable memory-erasure routine;
+calling `std::string::clear()` alone is not sufficient.
+
 #### `evaluate()`
-Executes the expression most recently compiled by `set_expression()`.
+Executes the expression most recently compiled by `set_expression()` or
+`set_protected_expression()`.
 *   **Signature:** `double evaluate();`
 
 #### `evaluate(const std::string&)`