Gauge v0.4.0 - Smart Performance + Explainability

Performance Mode
- Enabled by default.
- UNIT_STATS / COMBAT_RATING_UPDATE events are deferred while Gauge is hidden.
- Live stat caches refresh lazily the next time Gauge opens or an equippable tooltip is evaluated.
- No polling loop added; live data remains event-driven.
- /gauge perf prints lightweight cache/deferred-event diagnostics.

Tooltips
- Adds optional Why? explanation with up to two concise reasons.
- Adds Source agreement (0/4 through 4/4 exact provider matches).
- Reuses the existing cached Season 2 provider evidence.
- Keeps v0.3.8 tooltip restoration behavior.

Settings
- Toggle Why? lines.
- Toggle Source agreement.
- Toggle Performance Mode.

Existing v0.3.6-v0.3.8 caches and spike-reduction changes remain intact.
