[2.2] Composition → auto-populated components

[CSSFrancis]

The wave's headline feature. Enter the elements present; the model populates itself:

• EELS: background + an EELSCLEdge per ionisation edge falling in the measured energy range.
• EDS: a gaussian per X-ray line, with intensity ratios within a family tied so a family fits as one degree of freedom.

Then the user drags to adjust ([1.7]).

Part of #60.

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Plan: RELEASE_0_3_0_PLAN.md

[CSSFrancis]

Delivered in 0.3.0, merged to main via #89.

689e1a9 — feat(spectroscopy): composition -> auto-populated model

Closing from the merge rather than automatically: the commits cite issues as (#62) rather than Closes #62, so GitHub linked none of them (#89 reported 0 closing references).