pipeflow 0.3.0

New features

C++ DAG engine (src/dag.cpp): All graph operations (node/edge add/remove, topological ordering, reachability queries) run at C++ speed via Rcpp, eliminating R object overhead for dependency resolution.
New pip_* API: Functional API replacing the R6 Pipeline class — pip_new(), pip_add(), pip_run(), pip_replace(), pip_clone(), pip_bind(), pip_view(), pip_tag()/pip_untag(), etc.
Execution modes (auto/split/reduce/plain): Native support for map-reduce style workflows where steps split output into named partitions and downstream steps auto-map over them.
Dependency validation at definition time: pip_add(), pip_replace(), and pip_remove() fail fast on broken references.
New pkgdown-only article pipeflow vs targets (vignettes/articles/v07-vs-targets.Rmd) with an 18-row feature comparison table and benchmarks.

Legacy pipe_* functions and Pipeline R6 class are deprecated (preserved as aliases in R/aliases.R, R/pipelineR6.R).
pip_collect_out() no longer accepts grouped or by parameters. It returns a flat named list of step outputs. Use pip_view() with tags and manual list composition for grouped output.
The group column has been removed from the pipeline data.table. Steps previously using group = "..." in pip_add() should use tags = "...".

Comprehensive revision of all vignettes to match the new pip_* API.
Revised vignettes/v04-collect-output.Rmd to demonstrate tag-based grouping via pip_view() composition.
Updated README with a “Why use {pipeflow}” feature list, a short usage example, and a revised vs-targets summary.

Extensive test suite covering C++ DAG, pip_* API, aliases, execution modes, and recursive runs.
MIT license, updated CI workflows, dependabot config, kilo.json, lintr/styler config.