feat(vscode): draw the diagram in the pilot's style, filled from a server palette

[devin-ai-integration]

What and why

The diagram panel drew every diagram in the VS Code colour theme, while the DOT and PlantUML forms of the same view follow the pilot visualizer's Standard B&W and can be filled from eight colourblind-safe palettes. This PR gives the panel the same looks, selectable.

Style list. A Style <select> in the panel toolbar, backed by a new opensysml.diagram.style setting, picks the look of every diagram:

• theme (default) — as before, colours from the VS Code theme.
• pilot — the pilot's Standard B&W as docs/project/view-rendering-forms.md#style specifies it: white canvas, black sans-serif text, 0.5 px #181818 borders, square definitions / rounded usages, 1.5 px packages, dashed regions, bold name over a small italic «kind», 3 px arrowless connections, dashed flows, filled black pseudo-states.
• okabe-ito, tol-bright, tol-muted, tol-light, brewer-set2, brewer-dark2, viridis, cividis — that look filled by keyword family; a usage a lighter tint of its definition's colour, text black.

Changing the list writes the setting (at the level that set the value in force) and every open panel restyles and re-renders; the choice survives a panel restore.

Colour comes from the server, not the webview. Rather than reimplement the palette algorithm (family assignment, usage tint, WCAG-AA lightening, control-node exclusions) in TypeScript where it would drift, the render result carries it:

// internal/ir/view
type Fill struct{ Fill, Border string }
func (r *Rendering) Fills(palette Palette) (map[string]Fill, error)   // by node ID; absent = left B&W

// opensysml/render
{ "textDocument": …, "view": …, "palette": "okabe-ito" }          // request
{ "nodes": [{ "id": "n3", "kind": "part def", …, "fill": "#F6D9B1", "border": "#E69F00" }] }

Fills reads the same familyFills the DOT and PlantUML writers use, so the panel, a DOT export and a PlantUML export of one view agree hex for hex. The server advertises experimental.openSysmlRenderPalette; the panel asks for a palette only when it does, and against an older sysml-lsp draws pilot with a hint under the diagram saying why.

Canvas. Each box gets a class from its kind (package / definition / region / usage) so CSS can draw the pilot's shape rules, and a node's fill/border land as --node-fill / --node-border custom properties on its shape. Editing attributes (data-opensysml-id, data-kind), drag, drop-to-re-parent, waypoints and the context menu are untouched.

Not in scope. The pilot's -[thickness=5]- binding connectors: the renderer has no binding edge kind to style.

How it was verified

Go:

• internal/ir/view: TestFillsMatchDOT pins every fill/border to the colour the DOT form writes for the same node, across palettes and view kinds; TestFillsOutsideThePalette pins that B&W nodes, unfilled containers, tables and an empty palette yield nothing, and an unknown palette the registry's error.
• internal/frontend/lsp: render tests for a named palette (fields present, matching the artifact), no palette (fields absent from the JSON), and the capability in initialize.

Extension (npm run typecheck, npm test — 166 tests):

• style.test.ts: registry order, labels for every style, fallback of unknown setting/saved values, paletteOf/pilotLook.
• manifest.test.ts: the setting's default, enum and descriptions match the registry.
• protocol.test.ts: fill/border pass through normalizeRender; an older server's nodes carry neither.
• canvas.test.ts: box classes by kind, rounded usages, custom properties set only when both colours are present, identity attributes preserved, control nodes unfilled.

Gates run locally: go build ./..., go vet ./..., gofmt -l . (empty), make lint, go test ./... with OPENSYSML_REQUIRE_TRAINING_CORPUS=1 OPENSYSML_REQUIRE_PILOT_CORPORA=1, TestPilotLibraryXMI with the corpus downloaded, make docs-check, go run -C tools ./cmd/doc-counts -check, npm run build -- --production.

GUI verification in a real VS Code (Linux), against this branch's server and a v0.7.0 one for the fallback, is recorded in the comment below.

Checklist

• make test and make lint pass locally
• Tests added or updated for the change
• Documentation extended where it already covers the surface (docs/reference/lsp.md, editors/vscode/README.md, docs/project/view-rendering-forms.md, docs/internals/design/vscode-visual-modeling.md)
• Changelog entry added as changes/unreleased/diagram-pilot-style.added.md
• No gate count moved
• No internal work-item labels in the body, docs, or changelog

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[devin-ai-integration]

Verified in VS Code (Linux)

Recorded run of the extension built from this branch against its sysml-lsp, and against a v0.7.0 server for the fallback:

• Default is theme; the Style list offers all ten looks.
• pilot: white canvas, black text, bold names over a small italic kind, square definitions and rounded usages, thicker package border, dashed regions, thick arrowless connections, dashed flows; initial/final/choice/history nodes stay black and white, the final's ring intact.
• Okabe–Ito and viridis fill by family with visibly different colours; a usage is a lighter tint of its definition's; text stays black and legible.
• Choosing from the list writes opensysml.diagram.style to user settings; editing the setting restyles both open panels; Reload Window restores both in the chosen style.
• Editing under a palette: drag writes DiagramLayout coordinates, click reveals the source with the selection highlight, context-menu Rename rewrites references and labels.
• v0.7.0 server with viridis chosen: draws black and white with the hint "This language server draws no palette; update sysml-lsp to colour the diagram."; switching to theme clears it.

Theme	Pilot

Okabe–Ito	Viridis

Older server, palette chosen	State machine, viridis

Not exercised: macOS; the other six palettes beyond their list entries.

Seen, not from this PR: automatic interconnection routing can run an edge across a label (below). The layout and routing code is untouched here; worth its own issue.