Fix scalar * and / mutating the Solution in place (fixes #457)

[youdie006]

Fixes #457.

Problem

Standalone sol * factor and sol / factor mutated the original Solution and returned the same object. Root cause: __mul__/__truediv__ did self.volume *= factor; return self, and the volume setter rescales every component in place -- so sol * 2 doubled sol's volume and moles and returned an alias ((sol * 2) is sol was True), instead of a new scaled Solution. Reported by @jjstickel.

Fix

• __mul__ / __truediv__ now return a new scaled Solution, built via the existing faithful from_dict(as_dict(...)) round-trip and then scaling the copy's volume; the original is left unchanged.
• __imul__ / __itruediv__ are added to preserve the documented in-place *= / /= scaling behavior (mutate self, return self).
• __rmul__ is added so factor * sol works (commutativity).

This makes * and / consistent with Python numeric-type semantics, as @rkingsbury requested in the issue thread, while keeping the documented in-place *= / /= behavior intact.

Tests

• New test_multiplication_returns_new_solution: asserts sol * 2 is not sol, the original is unchanged, the returned Solution has 2x volume/moles, 2 * sol works (rmul), sol / 2 returns a new halved Solution, and sol *= 3 / sol /= 3 still mutate in place and return self.
• Verified red -> green (pre-fix the new test fails at assert doubled is not sol) and the full tests/test_solution.py suite passes with no regressions (test_arithmetic_and_copy included).

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This contribution was prepared with AI assistance and reviewed by me before submission.