[#15]:svarga:feature, GR.jl-style plot API (line/scatter) + Solarized themes

[steven-varga]

Closes #15.

GR.jl-style functional surface on plot:: + Solarized theming, and plot:: is now standalone (no bench/ dependency).

New (include/plot/)

• theme.hpp — plot::theme_t, solarized_dark/solarized_light, global current_theme + plot::theme(t) setter, plot::use{t} per-call override.
• gr.hpp — plot::line (single + multi-series w/ legend), plot::scatter, GR options xlabel/ylabel/xlog/ylog, shared panel/axis-scale/tick layout, theme resolution.

Changed

• meta.hpp — dropped the #include <bench/meta.hpp> bridge; self-contained plot::arg/plot::impl. grep -rn 'bench' include/plot/ is empty.
• tags.hpp, heatmap.hpp (gradient←theme, theme bg rect, honors use{}), all.
• examples line_chart.cpp (dark) + scatter.cpp (light); test/plot_gr.cpp; appended example/test targets; test/svg.cpp rect-count updated for the theme bg rect.

API

plot::line(os|file, xs, ys, opts...), multi-series plot::line(os, xs, {{"label",data},...}, opts...), plot::scatter(...). Options: title/xlabel/ylabel/xlog/ylog/width/height/margin/use.

Verification

Build clean (C++23, zero third-party); ctest 6/6; line_chart/scatter render themed SVG (dark #002B36, light #FDF6E3).

Note: absolute axis tick text uses text-anchor=start (canvas text() derives anchor from coord variant, not attr.align) — cosmetic, deferred per rework-phase guidance.

Base: staging.