Expand terminology glossary and add CONTEXT.md

[ecomodeller]

Summary

User-facing glossary (docs/user-guide/terminology.qmd):

• Adds four entries — Field, Geometry type (gtype), Baseline, Quantity — that were either entirely missing or only referenced inside other entries.
• Reframes Score as primarily a ComparerCollection concept (the cross-observation weighted average is what makes the result a single representative number); Comparer.score is documented as the degenerate single-observation case. Keeps the link target on ComparerCollection.score.
• Folds Performance into Skill as an alias instead of a separate entry — they were defined as synonyms.
• Fixes broken links in the Observation entry that pointed to PointModelResult / TrackModelResult instead of PointObservation / TrackObservation.
• Rewrites Timeseries to contrast point/track (timeseries) with Grid/dfsu (fields) and drop the inaccurate "line, or area" wording.
• Uses "flexible mesh (dfsu)" when referring to the geometry concept and "dfsu" when referring to the file format. Aligns with mikeio's CONTEXT.md, which is the upstream source-of-truth for this vocabulary. DfsuModelResult (class) stays — it is file-anchored. GeometryType.UNSTRUCTURED (enum) is flagged as legacy in CONTEXT.md but not renamed in this PR.
• Reorders entries by topic: framing (validation/calibration) → data shapes → matching → assessment → abbreviations.

Adds a dev-facing CONTEXT.md at the repo root:

• Mirrors the user-facing glossary, organised into Core entities / Geometry types / Skill assessment / Physical quantities.
• Additionally documents the Vertical alpha vocabulary (VerticalObservation, VerticalModelResult, SkillProfile, cmp.vertical accessor) which is deliberately kept out of the user-facing terminology page until the surface stabilises.
• Records conventions: z-axis direction (positive="up" default, see MIKE 3 convention), dfsu-vs-flexible-mesh distinction, the skill-vs-score granularity asymmetry, and the legacy UNSTRUCTURED enum name.

Test plan

• just docs renders without warnings on the new terminology.qmd anchors (#field, #geometry-type-gtype, #baseline, #quantity).
• Visually inspect the rendered glossary for correct icon/heading display and intra-page links.