feat: generic plotting engine — CTBase.Plotting + CTBasePlots (phase 1)

[ocots]

Summary

Phase 1 of the unified plotting engine. Adds a backend-agnostic intermediate representation (IR) — a weighted tree of titled axes carrying time series and decorations — plus the Plots.jl renderer.

• IR (src/Plotting): Series/HLine/VLine, Axes, Leaf/HBox/VBox, Figure. Pure data, no backend dependency.
• Domain-free building blocks: Panel (with per-component style), Stacked/Paired/Grid combinators.
• Lowering: Panel → Axes/tree, with time normalization, ylims guarding, deterministic leaf traversal.
• Backend contract: AbstractPlottingBackend + render/render! (stubs error; methods in ext).
• Plots.jl renderer (ext/CTBasePlots): Generalizes the CTFlows PlotEngine. Weighted layout via Plots.grid, per-series style with attribute filtering, decorations, margin/font semantics.

Decisions applied

• D1: Module name = CTBase.Plotting
• D2: Style vocabulary = minimal (color, linewidth, linestyle, alpha, seriestype, z_order) + backend_kwargs escape hatch
• D4: Stacked weights = :auto (proportional to cell count, reproduces CTModels behavior)

Testing

• 98 tests across 5 files: IR/combinators/lowering/heuristics (no Plots), contract + rendering (with Plots)
• Full CTBase suite + Aqua: ✅ green
• Format (blue style): ✅ clean
• CTModels/CTFlows: unchanged

Related

• Design report: CTModels.jl/.reports/dev/plot_engine_ctbase_report.md
• Phase 0 (reference matrix): PR Update README with latest ABOUT.md, INSTALL.md, CONTRIBUTING.md and badges #362
• Next: Phase 2 (CTFlows migration), Phase 3 (CTModels migration)

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