refactor: tighten prepare_expression — drop stale TODOs, expose original input

[hozblok]

Closes two TODOs in Formula::prepare_expression:

TODO	Resolution
csformula.cpp:185 // TODO check for forbidden symbols for complex: < or >, <= or >=.	Already implemented in the same function (lines 217-222 throw on < / > when is_complex_, and that catches <= / >= too via substring match). Just delete the stale comment.
csformula.cpp:195 // TODO cannot give back to user the original expression?	Resolved: add a parallel expression_original_ field, stash the unmolested user input before whitespace stripping / case lowering, expose via get_original_expression() + original_expression property.

Changes

• src/cpp/csformula/csformula.hpp — new private std::string expression_original_;, new public get_original_expression().
• src/cpp/csformula/csformula.cpp — prepare_expression stashes the input into expression_original_ before any normalization. Both TODO comments deleted.
• src/cpp/main.cpp — pybind11 binding gains a get_original_expression method and an original_expression read-only property on the Formula class.

Tests

tests/test_prepare_expression.py — 12 cases:

• Forbidden-symbol rejection on complex — parametrized over <, >, <=, >= on both sides of i (6 cases). All raise ValueError with the existing message.
• < allowed on real expressions — no i → real path, comparison is a valid binary op.
• get_original_expression preserves case — "X + Y" with case_insensitive=True → expression_ lowered to "x+y", expression_original_ keeps "X + Y".
• Preserves whitespace — " x + y " keeps spaces in the original even though expression_ strips them.
• original_expression property accessor — works the same as get_original_expression().
• Equal to get_expression() when no normalization happened.
• Updates on set_expression() — both stay in sync.

Full suite 400/400 (+12 new), 3 xfailed unchanged.