Surface unserved energy (balance slack) as a first-class dataset

[micahpw]

With the fixture now solved under DCPPowerModel; use_slacks = true (#4 / PR #9), the results store carries PSI's system balance slack variables:

• SystemBalanceSlackUp — fills generation shortfall: this is unserved energy / load shed
• SystemBalanceSlackDown — absorbs surplus; expected to stay ~zero in practice

Proposed work:

1. Expose slack-up as an unserved-energy dataset through the Scenario API (and decide its display-group treatment — e.g. an 'Unserved Energy' technology in dispatch stacks, conventionally rendered as a red band at the top).
2. Add a stressed fixture variant to actually exercise it — the standard RTS week solves with ~zero slack, so UE tests need a scenario that is genuinely short (e.g. a LOAD_SCALE env knob in generate.jl, or an outage week). Note any generate.jl change rotates the fixture cache key: bundle it with a deliberate refresh + baseline regen per the tests.yml procedure.
3. Regression-test the sign/direction convention: shortage shows up as positive slack-up, never as silently-clipped load.

This also feeds the units design (#8): unserved energy is exactly the kind of cross-tool comparison quantity (MWh of shed load) where unit and sign conventions differ between tools and must be normalized at the parser boundary.