Clarify constraint descriptor for v_old and root == anchor to indicate inclusive OR

[PlanetMacro]

In src/circuit.rs#L241, the constraint is currently described as:

"Either v_old = 0, or root = anchor"

However, this phrasing can be read as an exclusive-or (XOR), suggesting that not both conditions can be true at once.

To prevent confusion, the PR rephrased the comment/descriptor to

"v_old = 0, or root = anchor"

so it clearly indicates an inclusive-or rather than an exclusive-or. This will help auditors avoid misreading the condition as prohibiting both v_old == 0 and root == anchor from being true simultaneously.