feat(traces): add Splom + Parcats trace types

[jqnatividad]

Summary

Adds two typed traces to the fork, following the established trace-struct recipe (mirroring sankey.rs and histogram2d.rs). Stacked on plotly#423 — base is feature/trace-type-additions; retarget to main once that merges.

• Splom<V> (scatter-plot matrix) — SplomDimension/SplomAxis/SplomDiagonal sub-structs, showupperhalf/showlowerhalf controls, reuses the common Marker. Plotly.js auto-generates the N×N axis grid, so no explicit xaxis/yaxis assignment.
• Parcats<V> (parallel categories) — ParcatsDimension/ParcatsLine sub-structs, counts per-path weighting, domain-based (like Sankey). Enums: ParcatsLineShape, ParcatsArrangement, ParcatsHoverOn, ParcatsSortPaths.

Details

• PlotType::{Splom, Parcats} serialize to "splom"/"parcats" via the existing rename_all = "lowercase" (no explicit rename needed, same as Sankey).
• Module + top-level re-exports wired through traces/mod.rs and lib.rs; CHANGELOG.md updated under [Unreleased] → Added.
• Every attribute was verified against the bundled Plotly.js 3.7.0 schema, including the two details the design doc flagged as unconfirmed: parcats hovertemplate is genuinely trace-level, and arrangement also accepts "fixed" (included for schema completeness).

Note for reviewers

SplomAxis::type reuses the shared AxisType enum, which is slightly wider than splom's allowed set (linear/log/date/category) — it also carries -/multicategory. This follows the "reuse common types" convention; Plotly.js coerces invalid values. Happy to swap in a narrower splom-specific enum if preferred.

Testing

• cargo test -p plotly --features all — 371 unit + 36 doctests pass (includes to_json() round-trip + doc examples for both traces)
• cargo clippy --features all -- -D warnings -A deprecated — clean
• cargo doc — no new warnings

🤖 Generated with Claude Code

[jqnatividad]

Superseded by the standalone upstream PR plotly#424 (based directly on main, independent of plotly#423). Branch feature/splom-parcats-traces is kept as the git-dependency source for qsv until the upstream trace additions are released.