feature - implement RFC 021 generator foundations (#38)

[dannymeijer]

Summary

Adds the first RFC 021 generator/table-valued function foundation on top of the RFC020 branch. This slice introduces registry-backed generator applications for the explode family, keeps generators distinct from scalar expressions and aggregate measures, and lowers them through the current Substrait ExtensionSingleRel relation-extension boundary.

Refs #38.

Stack order: merge #44, then #45, then #46, then this PR.

Type of change

• Bug fix
• New feature
• Refactor / maintenance
• Documentation
• CI / tooling
• RFC (adds/updates docs/rfcs/*)

Area(s)

• Package & tests
• Specification (RFCs)
• Documentation
• Automation & repo config
• Other

Key details

• User-facing behavior: Adds generate(...) plus explode(...), explode_outer(...), posexplode(...), and posexplode_outer(...) helper applications with required output aliases. The existing zero-arg DataSet.explode() remains as a compatibility marker.
• Internals: Adds GeneratorApplication, generator registry metadata, Prism Generate nodes, generator-aware output-column inference, relation-extension lowering, and unique relation extension URN anchors when multiple extension relations appear in one plan.
• Risks: This is an RFC021 foundation slice, not the full RFC. inline, inline_outer, stack, portable table-valued flatten, query-block syntax, and DataFusion table-valued execution remain future work.

Testing / verification

• make ci (or make fmt-check, make build, make test)
• Manual verification described below

Manual verification notes:

• incan test tests/test_generator_functions.incn
• incan test src/generator_builders.incn
• incan test src/functions/generators
• incan test tests/test_prism.incn
• incan test tests/test_dataset.incn
• incan test tests/test_function_registry.incn
• incan test tests/test_substrait_plan.incn
• make fmt
• make fmt-check
• make test-style
• make registry-metadata
• make build
• make test
• make smoke-consumer
• make pre-commit

Docs impact

• No docs changes needed
• Docs updated
• Docs follow Divio intent (tutorial/how-to/reference/explanation) where applicable

If docs updated:

• Link(s): docs/language/reference/functions/generators.md, docs/language/reference/dataset_methods.md, docs/rfcs/021_generator_table_functions.md, docs/release_notes/v0_1.md

Checklist

• I kept public docs user-focused and moved internals to contributing docs when appropriate
• I avoided duplicating canonical install/run instructions in multiple places
• I added/updated tests where it materially reduces regressions