feat(plotting): Makie backend parity with the Plots backend

[ocots]

Brings CTBaseMakie (proof of concept, render only) to feature parity with CTBasePlots.

What changed

ext/CTBaseMakie.jl:

• seriestype dispatch — :steppost → Makie.stairs!(…; step=:post), :scatter → Makie.scatter!, :path → Makie.lines!. Constant-interpolation controls now render as steps.
• z_order — series drawn back-to-front using the same _z_rank ordering as the Plots backend.
• Decorations — HLine / VLine drawn via Makie.hlines! / vlines! with style translation (box bounds, t0/tf markers).
• User kwargs — _partition_user splits into (series_user, axes_user); axis attributes (legend, ylims, grid/scale/ticks) forwarded to every cell, unknown keys dropped. Makie exposes no attributes(:Series) analogue, so the split uses curated whitelists (_SERIES_USER_KEYS / _AXIS_USER_KEYS).
• render! overlay — real implementation: targets existing Makie.Axis blocks by deterministic leaf order; an empty target figure is filled as if by render (new _render_into! helper, shared with render).

src/Plotting/contract.jl — MakieBackend docstring drops the proof-of-concept caveat.

test/suite/plotting/test_contract_makie.jl — POC assertions replaced by parity assertions mirroring test_contract.jl on the same _figure() IR (overlay keeps axis count, decorations become HLines/VLines, :steppost becomes Stairs, forwarded attributes reach the axes).

Follow-up commits on this branch

• 1a2624f — :split cells hardcode legend=false; a user label= (or an IR series label) now turns the legend back on, on both backends, so overlaid solutions can be told apart.
• 1b71e9e — docs: name the Makie backend in the Architecture Overview + the "no backend loaded" note; new "User Attributes: Series vs Axis" section in the plotting guide documenting the per-backend kwarg split.

Docs

• CairoMakie added to docs/Project.toml; make.jl loads it and registers :CTBaseMakie; the plotting guide gains an executed Makie @example; CTBaseMakie appears on the API reference internals page.
• make.jl warnonly gains :external_cross_references. The self-referencing InterLinks "CTBase" entry cannot resolve symbols added since the last published inventory (here MakieBackend, latent since feat(plotting): add Makie backend (proof of concept) #539) on a fresh checkout; per the Handbook self-reference pattern the first build completes-with-errors to produce objects.inv, then a second build is clean. Matches CTModels.jl, which already sets this.

Testing

Last full local run 2026-08-28 (1b71e9e):

• test/suite/plotting — green
• full suite — 120460 pass
• docs — clean (2-pass on a fresh checkout, see above)

Part of control-toolbox/CTModels.jl#408.

🤖 Generated with Claude Code

[ocots]

Updates since the PR was opened

• 1a2624f fix(plotting): a user \label=` shows the legend on a :split cell— regression fix affecting **both** backends._lower_splithardcodeslegend=false; a user label=(or an IR series label) now turns the legend back on so overlaid solutions can be told apart. Verified:plot(sol; layout=:split, label="tat")` → legend on every cell (Plots + Makie); no label → no legend. Full suite 120460 pass.
• 1b71e9e docs: name the Makie backend everywhere the engine's backends are listed — Architecture Overview diagram/prose + the "no backend loaded" note were still Plots-only; brought level. Adds a "User Attributes: Series vs Axis" guide section explaining how each backend partitions user kwargs (Plots introspects Plots.attributes(:Series); Makie uses curated whitelists and drops unknown keys). Docs build clean.