Add ColorSequence and Plotly Express qualitative color sequences

[iynehz]

Fixes #396

Adds plotly::color::qualitative with a ColorSequence type and all 19 sequences from Plotly.py's plotly.colors.qualitative module (px.colors.qualitative), plus reversed (*_R) variants matching Python's *_r lists.

API

• ColorSequence — len(), is_empty(), const get() with modular indexing, reversed(), iter() (non-allocating Iter), as_vec(), IntoIterator
• Constants: PLOTLY, D3, G10, T10, Alphabet, Dark24, Light24, Set1, Pastel1, Dark2, Set2, Pastel2, Set3, Antique, Bold, Pastel, Prism, Safe, Vivid (+ each *_R)
• ALL: &[(&str, ColorSequence)] and case-insensitive by_name() for name-based lookup

Notes

• Values taken from plotly.py 7.1.0; rgb(...) entries converted to equivalent uppercase #RRGGBB hex
• Accent / Paired intentionally omitted (not re-exported by qualitative)
• Docs: https://plotly.com/python/discrete-color/#color-sequences-in-plotly-express

Checks

• cargo fmt (new file clean under nightly)
• cargo clippy --features all -- -D warnings -A deprecated
• cargo test -p plotly (387 unit + 34 doc, including 9 new qualitative unit tests and 2 new doc examples)

[iynehz]

CI note: Clippy, Tests, examples, coverage, typos are green.

Rustfmt and semver fail for reasons that also fail on current main (verified on a clean upstream/main worktree):

• Rustfmt: current nightly rewraps comments (wrap_comments = true); 22+ pre-existing files diff. The new qualitative.rs is clean under nightly rustfmt.
• semver: cargo-semver-checks with the action’s feature set hits plotly_static’s “only one of geckodriver/chromedriver” compile_error while building rustdoc — also fails on main. Has been red on recent main runs (e.g. since July).

This PR only adds public items (plotly::qualitative), so it should be minor-compatible once those checks are fixed on the base branch.

[iynehz]

Updated path to nest under the existing color module (same PR, force-pushed):

use plotly::color::qualitative;

qualitative::DARK24.get(0)
plotly::color::qualitative::ColorSequence

• common/color.rs → common/color/mod.rs (+ pub mod qualitative)
• common/qualitative.rs → common/color/qualitative.rs
• Removed top-level plotly::qualitative re-export (it was never on a release)

Still additive for plotly::color::{Rgb, NamedColor, Color} users.