[2.1] exspy extra, signal-type gating, microscope parameters

[CSSFrancis]

• eels extra wired up; actions gated with the new requires_package key plus the existing signal_types gate (EELS, EDS_TEM, EDS_SEM).
• Microscope-parameter panel: beam energy, collection and convergence angle, live time / real time for EDS.

These are not optional metadata — an EELS or EDS fit is meaningless without them, so the panel must prompt when they are absent rather than silently fitting with defaults.

Part of #60.

---

Plan: RELEASE_0_3_0_PLAN.md