feat: add max-share frequency-band identification #145

What to build

Add a max-share identification scheme that finds the structural shock explaining the largest share of a named variable's variation over a user-selected frequency band. The implementation should operate per posterior draw, normalise shock signs deterministically, and return structural matrices through the existing identification pipeline. A climate example should distinguish low-frequency forced warming from interannual internal variability without claiming that frequency separation alone establishes causality.

Acceptance criteria

• Users can select a target variable and valid frequency band and receive sign-normalised structural draws with stable shock labels.
• The optimisation objective and resulting variance shares match an independent synthetic or published reference case across posterior batches.
• IRF and FEVD outputs compose through IdentifiedVAR, and the documentation explains frequency units, normalisation, and identification limits.

Blocked by

None - can start immediately.