doc: Prism theme audit — fidelity review across diagram families

[Copilot]

Prism's white-fill + 4-stop rainbow border-gradient identity renders correctly through the standard AppendGradientDefs path but is silently broken in several diagram types that either set render:noGradient=true with palette-derived fills or use NodePalette as direct fill colors — both of which collapse to white-on-white in Prism.

Audit document: doc/prism-audit.md

Full findings with SVG fixture evidence and code-level root causes for every diagram type in the review set.

Categorization

Rating	Types
A — renders well	Mermaid flowchart, subgraph, mindmap, class, sequence, state, architecture, block, venn, timeline; conceptual chevrons, funnel, pyramid, cycle, radial, tree
B — mutes identity	Mermaid xychart — bars get accent-tint fills via TryResolveXyChartColors, bypassing NodePalette; Prism border gradient is present
C — bypasses Prism	Conceptual snake, target, tablist (all 3 variants)

Category C root causes

• Snake (DefaultLayoutEngine.Snake.cs): node.FillColor = palette[i] and segmentColors.Add(palette[i]) use NodePalette directly. In Prism every entry is #FFFFFF → invisible tube and circles on white background.
• Tablist — all variants (DefaultLayoutEngine.TabList.cs): all three layouts set render:noGradient=true and derive accent fills from Vibrant(palette[i], 2.x). Vibrant of pure white is still white → accent blocks, title bars, and number tabs are entirely invisible.
• Target (DefaultLayoutEngine.Target.cs): ring and center nodes carry render:noGradient=true with explicitly set solid colors from ThemePaletteResolver.BuildRingColors; AppendTargetRingNode/AppendTargetCardNode never consult theme.BorderGradientStops.

Proposed follow-up issues (detailed in audit doc)

1. Critical — Snake: detect monochrome palette, fall back to BorderGradientStops as segment colors
2. Critical — Tablist: when palette is achromatic, substitute AccentColor / first BorderGradientStop
3. High — Target: emit gradient ring stroke from theme.BorderGradientStops when available
4. Medium — XYchart: use BorderGradientStops as series palette when UseGradients=false && UseBorderGradients=true

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