docs: Makie plotting page (phase L)

[ocots]

What

Adds results/plot-makie.md, the contrast page for the Makie backend beside the existing Plots reference (results/plot.md). Plan: .reports/campaign/L-makie-docs.md.

Why this shape

Hub + contrast, not a generic page or two parallel references — decided on three measurements:

• OptimalControl re-exports Plots' plot/plot! unconditionally, so a session with a Makie backend loaded has no backend-neutral call to write: bare plot raises UndefVarError. The page opens on exactly that error, live.
• Keyword parity between the two backends is total (every keyword on results/plot.md measured to work on Makie.plot), so a parallel reference would only drift. The new page carries the delta only.
• flows/*.md needs no edit — both flow trajectory kinds already plot correctly through the same call.

The core idea the page teaches

plot/plot! are a Plots-shaped convenience API that build a whole figure in one call — not the backend itself. Customising means dropping to the backend's own commands through the panel: plt[i] on Plots, f.content[i] on Makie.

On Plots, plot! does double duty (overlay a solution and add a native series, same function). On Makie the two roles split — Makie.plot!(f, sol) is solution overlay, lines!(ax, …) is a native addition — and Makie.plot!(ax, sol), the call a Plots reader reaches for, fails. The page shows it failing, then the fix.

results/plot.md gains a short "Choosing a backend" pointer plus a native-annotation example in its existing Custom subplots section, so both pages state the same idea in their own backend.

Build plumbing

SVG rendering for Makie figures already shipped in #933 (import CairoMakie, not using — avoids the exact plot/plot! clash this page teaches, biting at build level). This PR only adds the page and:

• a nav entry after "Plot" in docs/make.jl;
• CTModelsMakie / CTFlowsMakie registered in the extension-docstring loop, so the page's Reference section resolves (spelled CairoMakie.Makie.plot(…) — Makie is not a bare name in Main under import CairoMakie).

Verified

• Build exit 0.
• 0 unresolved @ref (baseline since docs: render CairoMakie figures as SVG, like Plots #933) — unchanged.
• The only Error:-level log lines are pre-existing @extref noise already under warnonly=[:cross_references, :external_cross_references] (Phase D): CTModelsMakie's own docstrings carry the same broken upstream @extref to Plots.plot(::CTModels.Solutions.Solution) that results/plot.md already tolerates via CTModelsPlots. +2 instances, same class, not a new category.
• No .png in docs/build/1/results/ beyond the static site logo (#933's invariant holds).
• 7 SVG figures render on the new page, sizes consistent with content (the 5-panel constrained-problem figure is largest, the single-panel custom figure smallest); the annotated "after" figure visibly contains the "switch" text label and 8 dashed strokes (control bounds + switching marker) — confirmed in the SVG source.
• docs/src/assets/ unchanged (no compat move in this phase).

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