feat(mbpt): square-root-normalized δr, δl projector operators

[ajay-mk]

Adds δr and δl square-root-normalized excitation/deexcitation projector and the Normalization machinery they need. Uses Power (#513) to represent the square roots.

Motivation: In UCC-EOM the eigenproblem must be Hermitian, which requires the normalization to be split symmetrically between the bra (projector) and ket (operator) sides. To get the standard 1/(c!a!) normalization, it's square root is applied on each side (in both projector and operator). The total stays 1/(c!a!) = 1/(k!)² for a particle-conserving rank-k block; = 1/k! per side.

Normalization conventions

Default     →  1/(c! a!)          (standard operator builders)
Implicit    →  no prefactor       (A, S, definition carries normalization)
SquareRoot  →  √(1/(c! a!))       (δr, δl)

Changelist

• Normalization enum is moved to mbpt scope and a new member SquareRoot is added.
• OpMaker supports SquareRoot normalization
• get_unique_indices now handles Power correctly
• Adds an optional normalization argument to r, l, R, L (defaulting to Default) and P, A, S (defaulting to their intrinsic Implicit)
• Defines δr/δl as SquareRoot-normalized P: no need to make new definitions, they have the same structure.
• eom_r/eom_l now build R/L with SquareRoot normalization and project with δl/δr instead of bare P
• No behavior change for existing r/l/R/L/P/A/S callers, the new Normalization argument defaults preserves existing behavior.

[Krzmbrzl]

In the literature I have often read the term Symmetric for this case. Not sure if this should be preferred here though as I guess Normalization itself doesn't imply the presence of a LHS and RHS state/component. 🤔