Pen catalogue: read units, per-pen scale, visibility and multiple groups

Why

A pen carries only PenId, Name, Group, Color, LineStyle (SemiPlot.Core/Trends/Pen.cs:3-7). The catalogue query never selects unit (SemiPlot.DataSource.Postgres/ArchiveStatements.cs:21-25), a pen has one group, its scale is stored nowhere, and there is no stored visibility. Semiteq/SemiBase#9 extends the schema; this is the client half.

Wanted

• Pen grows Unit, Enabled, ScaleMin, ScaleMax (both nullable, NULL meaning autoscale) and Groups as a list instead of a single Group.
• ArchiveStatements.PenCatalog reads the new shape: semiplot_tags joined to semiplot_pen_groups and semiplot_groups. The current ORDER BY coalesce(group_name, ''), name has to go, because a pen now has several groups and no single sort key.
• PostgresDataProvider maps the rows (PostgresDataProvider.cs:437-462). It already logs and falls back on an unknown line_style; color gets the same treatment instead of today's new Color("") on a NULL row (Chart/TrendChartViewModel.cs:367, Legend/LegendConverters.cs:11-12).
• The startup probe refuses a database whose semiplot_meta.schema_version is older than this client, reporting it in the existing startup-failure window rather than throwing on a missing column.
• The bench seeder fills the new tables. SyntheticPenCatalog already holds MinValue and MaxValue per pen with nowhere to store them (SemiPlot.Tools.ArchiveSeeder/SyntheticPenCatalog.cs:24-36) and assigns colours round-robin (:14-18). It gains units per group and writes the group tables through TagCatalogWriter (TagCatalogWriter.cs:10-19).
• Legend headers come from group membership. A pen in two groups appears under both. A pen with no group shows under one "Ungrouped" header.
• Docs: docs/architecture/postgres-instance.md (the table description, and the line at :70 recording that no query reads unit), data-integration.md, bench.md.

Notes

• The catalogue is read once at startup (App.axaml.cs:121-124) and Pen is immutable. Re-reading a changed configuration in a running viewer belongs to #67.
• The stored per-pen scale is consumed by #66, which replaces the group axis key.