[datlechin]

Summary

• ER diagram toolbar's export button is a menu: PNG, SVG, PDF, Mermaid, Graphviz DOT, and Copy as Mermaid.
• Node and edge renderers draw through ERDiagramCanvas: a CGContext canvas for the screen, PNG and PDF, ERDiagramSVGCanvas for SVG. Every export comes from the same drawing calls.
• PDF: one page sized like the PNG, text stays searchable text.
• SVG: shapes and <text>, truncated like the canvas. SF Symbols have no public vector outline, so each one is a PNG in <defs>, placed with <use>.
• Mermaid (.mmd): quoted table names, types and columns fitted to the Mermaid 11.6+ lexer (character varying(255) reads character_varying(255), pk reads _pk), PK and FK keys, the canvas's marker at each end. Copy as Mermaid wraps it in a mermaid fence.
• Graphviz DOT (.gv, since .dot resolves to Word's template type): HTML-like label per table, a full-width port row per column with generated port ids, crow, tee and odot arrowheads.
• Compact mode and junction collapse carry into every format. Text formats sort tables by name and edges by table and column.
• DiagramImageExporter is now DiagramExporter, with one save for every format.
• Fixed: key, link and junction icons drew upside down on the canvas and in the PNG. NSImage.draw(in:from:operation:fraction:) ignores the flipped context; the draw now passes respectFlipped: true.
• Fixed: the graph builder read foreign keys in dictionary order, so edge order, and port order on ties, changed per launch.

Test plan

• New: ERDiagramMermaidWriterTests, ERDiagramDOTWriterTests, ERDiagramVectorExportTests.
• Fail on main: ERDiagramGraphBuilderTests/edgesFollowTableOrder(), ERDiagramVectorExportTests/symbolsDrawUpright().
• Existing ER and query plan suites pass on the canvas refactor, including the PNG pixel tests in ERDiagramThemeTests.
• A 31-table schema exported collapsed, expanded and compact, light and dark: Mermaid parses with the 11.10 and develop grammars, DOT renders with Graphviz 12.1.2 with every port resolved, SVG passes xmllint and renders in WebKit and rsvg with the PNG's geometry, PDF renders as one page.

Fixes #2519