fix(masterclass): emit result-recorded mass in GeV (#1042)

[coder-Yash886]

Closes #1042

Summary

Fixes #1042 by ensuring the result-recorded event bus payload emits invariant mass values in GeV rather than MeV.

Problem

When recording an invariant mass result in the masterclass panel (e.g., ~91 GeV for a Z candidate), the component emitted result-recorded with mass: this.liveMass which is in MeV (~91187 MeV). The histogram panel subscribed to this event and added the raw value without unit conversion. Since all histogram configs (ATLAS_MASS_HISTOGRAM, J/ψ, wide configs) use GeV ranges (e.g., 20–120 GeV for Z), all recorded results were going to overflow, leaving the visible histogram plot empty.

Solution

1. Emitter Conversion: Converted mass to GeV (result.mass / 1000) when emitting result-recorded in MasterclassPanelOverlayComponent.recordResult(). Internal state (massResults) remains in MeV for panel UI and export consistency.
2. Event Bus Contract: Documented that result-recorded payload emits mass in GeV in event-display.ts.
3. Testing: Added unit test in masterclass-panel-overlay.component.test.ts to verify result-recorded emits mass values converted to GeV.

Related Issues

[coder-Yash886]

@EdwardMoyse Please review the pr when you have the free time

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