feature - implement RFC 021 generator table functions (#38)

[dannymeijer]

Summary

Implements RFC 021 generator and table-valued functions after RFC 020 has merged into main. This PR adds the full portable generator surface for explode, explode_outer, posexplode, posexplode_outer, inline, inline_outer, flatten, and stack, keeping generators distinct from scalar expressions and aggregate measures while lowering them through Substrait relation-extension metadata.

Refs #38.

Stack order: #44, #45, and #46 are merged; this PR targets current main directly.

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 generator helpers for explode(...), explode_outer(...), posexplode(...), posexplode_outer(...), inline(...), inline_outer(...), portable table-valued flatten(...), and stack(...). Generated aliases are explicit and validated. The zero-arg DataSet.explode() remains a lower-level extension-boundary operation rather than the RFC 021 generator surface.
• Internals: Adds GeneratorApplication, generator registry metadata, Prism Generate nodes, payload-backed Substrait relation-extension lowering, generator-aware output-column inference, and DataFusion execution for root and composed generator plans.
• DataFusion bridge: Uses Substrait as the semantic boundary, then materializes generator extension relations into execution-only temporary tables where the stock DataFusion Substrait consumer cannot consume InQL generator extensions directly.
• RFC status: Marks RFC 021 implemented. Query-block syntax itself is owned by RFC 003; when that surface lands, generator clauses should lower to this GeneratorApplication model.

Testing / verification

• make fmt INCAN=/Users/danny/Development/encero/incan/target/debug/incan
• incan test tests/test_session_generators.incn (10 passed)
• incan test tests (143 passed)
• make fmt-check INCAN=/Users/danny/Development/encero/incan/target/debug/incan
• make test-style INCAN=/Users/danny/Development/encero/incan/target/debug/incan
• make registry-metadata INCAN=/Users/danny/Development/encero/incan/target/debug/incan (87 helpers)
• make build INCAN=/Users/danny/Development/encero/incan/target/debug/incan
• make test INCAN=/Users/danny/Development/encero/incan/target/debug/incan (143 passed)
• make smoke-consumer INCAN=/Users/danny/Development/encero/incan/target/debug/incan
• make pre-commit INCAN=/Users/danny/Development/encero/incan/target/debug/incan (143 passed)
• git diff --check
• Redundant URI accessor sweep: no remaining *_extension_uri() helper call sites in src, tests, or docs

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/functions/nested.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