Add Splom and Parcats trace types

[jqnatividad]

Summary

Adds two typed traces to the plotly crate, following the established trace-struct recipe (FieldSetter derive + skip_serializing_none, mirroring sankey.rs and the other domain/matrix traces):

• 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 callers don't assign explicit xaxis/yaxis ids.
• Parcats<V> — parallel categories. ParcatsDimension/ParcatsLine sub-structs, counts per-path weighting (so aggregated tuples can be passed instead of one row per record), domain-based like Sankey. Enums: ParcatsLineShape, ParcatsArrangement, ParcatsHoverOn, ParcatsSortPaths.

Both are additive, non-breaking public API.

Details

• PlotType::{Splom, Parcats} serialize to "splom"/"parcats" via the existing rename_all = "lowercase" (same as Sankey, no explicit rename).
• Module + top-level re-exports wired through traces/mod.rs and lib.rs; CHANGELOG.md updated.
• Every attribute was cross-checked against the bundled Plotly.js 3.7.0 schema. Notably: parcats hovertemplate is trace-level, and arrangement accepts perpendicular/freeform/fixed (all three included).

Note for reviewers

SplomAxis::type reuses the shared AxisType enum, which is slightly wider than splom's documented set (linear/log/date/category) — it also carries -/multicategory. This follows the crate's "reuse common types" convention; happy to introduce a narrower splom-specific enum if you'd prefer.

Testing

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

🤖 Generated with Claude Code

[jqnatividad]

Reopening as a stacked PR on top of #423 instead of a standalone branch off main.