feat(palette): adopt imprint — new categorical palette + named API + frontend rebuild

[MarkusNeusinger]

Summary

Replaces the previous variant D palette (PR #7617) with imprint — anyplot's new 8-hue colourblind-safe categorical palette + 3 semantic anchors (amber for warning, theme-adaptive neutral and muted). Tuned for warm-paper rendering, validated against deuteranopia / protanopia / tritanopia, sits in the academic-publishing family next to Okabe-Ito, Paul Tol "muted", and ColorBrewer Set2.

Full design rationale: docs/reference/palette-variants-v3/decision-rationale.md. Earlier variants (v1, v2, expert reviews) preserved under docs/reference/palette-variants-*/ for design archaeology.

What's in this PR

Phase 1 — Backend foundation (core/)

• NEW core/palette.py — single source of truth: 8 categorical hexes in hybrid-v3 sort order, 3 semantic anchors, named API (palette.green, palette.semantic.bad, …), sequential cmap imprint_seq (green→blue) + theme-adaptive diverging cmap imprint_div_{light,dark} (red↔midpoint↔blue), matplotlib registration.
• core/images.py refactored — drops old 7 ANYPLOT_* constants, re-imports from core/palette.py. LIBRARY_COLORS rebuilt with library-brand-match strategy.

Phase 2 — Prompt system (18 files)

• prompts/default-style-guide.md — full Color Philosophy rewrite with 8-hex hybrid-v3 table, new Semantic anchors section, series-count guidance with concrete ΔE_CVD numbers, optional outline pattern.
• 11 library prompts (matplotlib / seaborn / plotly / bokeh / altair / plotnine / pygal / highcharts / ggplot2 / letsplot / makie) — ANYPLOT_PALETTE lists updated to 8 hexes + ANYPLOT_AMBER, cmaps renamed with new endpoints.
• Quality + workflow prompts — VQ-07 scoring extended to positions 1-8 + 3 anchors, legacy variant-D hexes listed as auto-reject signals.

Phase 3 — Style guide doc

• docs/reference/style-guide.md Section 4.1 rewritten as "The imprint Palette" with hue table, semantic-anchors table, hybrid-v3 sort rationale. Status colours (§4.5) and plot-only colours (§4.6) remapped onto imprint. CSS variables + Python implementation reference updated.

Phase 5a — Frontend palette page (/palette)

• app/src/pages/PalettePage.tsx completely rebuilt across 5 iterations: real chroma wheel (CSS conic-gradient OKLCH + chroma fade) with hover tooltips, compare-with toggle (Okabe-Ito / Tol muted / ColorBrewer Set2 / anyplot previous), compact 4×2 slot grid with click-to-copy hex, sort toggle (imprint default ↔ CVD-optimal max-min) with inline trade-off info, semantic anchors cards, continuous cmaps gradient strips, copy snippets in 4 languages (Python / R / Julia / JS) with OKLCH toggle, collapsible WCAG audit + palette history.
• theme/index.ts — colors.okabe → colors.imprint, semantic colours remapped.
• styles/tokens.css — --ok-* → --imprint-*, code-syntax theme rebuilt for both light and dark.
• components/PaletteStrip.tsx — 8-hex imprint set, optional hexes prop.

Phase 5b — Frontend consumers

• LandingPage.tsx — CLUSTER_PALETTE + 7-Okabe swatch row swapped (now click-to-copy), palette_okabe_ito analytics event gone.
• MapPage.tsx — CLUSTER_COLORS rebuilt with 8 imprint hexes.
• AboutPage.tsx, ScienceNote.tsx — palette prose reframed around imprint.
• CodeShowcase.tsx, CodeHighlighter.tsx — syntax hexes updated, okabeItoTheme → imprintTheme.
• LandingPage.test.tsx — drops the gone Okabe-Ito tracking-event test.

Final audit sweep

• docs/reference/plausible.md (gone event row), .serena/memories/style_guide.md (Color section), scripts/style-variants.yaml (palette_tableau map), scripts/palette-analysis.py + docs/reference/anyplot-landing-mockup.html (HISTORICAL markers). Historical / comparison references (Okabe-Ito as family-neighbour, variant-D hexes as auto-reject signals in VQ-07, palette-variants exploration scripts) preserved intact.

Intentionally NOT in this PR (Phase 4)

7 plot implementations under plots/**/implementations/python/*.py still hardcode the old variant-D palette inline. Per the design discussion these stay untouched and will pick up imprint via the next /regen cycle, when the new prompts kick in.

Validation

• yarn tsc --noEmit — clean
• yarn lint — 0 errors (2 pre-existing warnings, neither palette-related)
• yarn test --run — 507 / 507 passing
• Backend smoke-test confirms named-API + cmap registration
• Visual smoke tests via chrome-devtools on /, /palette, /map, /about

Test plan

• Verify /palette page renders the chroma wheel, palette grid, semantic anchors, cmap previews, collapsibles
• Try the compare-with toggle (Okabe-Ito / Tol muted / Set2 / anyplot previous) — overlay rings appear on the wheel
• Try the sort toggle (imprint default ↔ CVD-optimal) — grid + strip + copy snippet all reorder
• Click a swatch in the grid — copies hex to clipboard, swatch flips to "copied ✓"
• Switch OKLCH toggle in the copy section — all 4 language snippets switch notation
• Expand the WCAG audit and palette history collapsibles
• Verify LandingPage palette section + map cluster preview render with imprint colours
• Sanity-check dark mode on / and /palette
• Watch Cloud Build: OG image regeneration should pick up the new LIBRARY_COLORS

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