build: remove DataFusion from default builds

[nmrtist]

Summary

• use the reference table executor for development and shipping builds
• remove DataFusion and Substrait from default workspace and PR validation paths
• retain the DataFusion implementation and add an explicit cargo datafusion-build maintenance entry point
• generate shipping licenses from the reference-only Windows app dependency graph
• update CI, contributor guidance, repository rules, and UI wording

Behavior

Default app, CLI, root workspace, release, and PR builds no longer compile or link DataFusion.

DataFusion remains available explicitly:

cargo datafusion-build
cargo test -p plotx-datafusion
cargo test -p plotx-core --features datafusion
cargo test -p plotx-substrait

Cargo.lock continues to contain DataFusion because the dormant crates remain workspace members.

CI

PR CI now:

• checks both frontends with default features
• runs Clippy and tests across the workspace except plotx-datafusion and plotx-substrait
• audits the same reference-only dependency graph
• no longer installs protoc
• no longer validates the DataFusion backend

Verification

• cargo pr-check