feat: implement InQL RFC 001 — dataset hierarchy and DataSet API

[dannymeijer]

Summary

This PR implements InQL RFC 001 in the InQL package: the dataset type hierarchy and programmatic relational API that downstream RFCs (query {}, Substrait, execution) build on.

The library now exposes the full trait ladder—DataSet[T] → BoundedDataSet[T] / UnboundedDataSet[T] → DataFrame[T], LazyFrame[T], DataStream[T]—with the eight DataSet instance methods spelled in the RFC using Incan’s Self-based shipped signatures, plus pub::inql.functions aggregate helpers for the .agg(...) import model. Method bodies are stubs that fail at runtime until execution lands (InQL RFC 004); the contract is types, exports, and compile-time structure.

Docs catch up with Implemented / Shipped in: 0.1.0, the contract-complete checklist, reference and explanation pages under docs/language/, examples under examples/, and RFC index notes. Static capability gating (compile-time rejection of invalid ops on unbounded carriers) remains Incan typechecker work—tracked as incan #187, with InQL #10 to revisit package/docs when that exists.

Closes #2

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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: Authors can depend on inql 0.1.0 and from pub::inql import DataSet, BoundedDataSet, UnboundedDataSet, DataFrame, LazyFrame, DataStream, compose signatures against the hierarchy, call the relational method surface on carriers, and import total / count_rows from pub::inql.functions for the aggregate story RFC 001 describes. No real query execution or I/O yet—that is RFC 004 and friends.
• Internals: New modules src/dataset.incn, src/functions.incn; src/lib.incn re-exports; tests/test_dataset.incn exercises exports and trait assignability; examples/* plus examples/README.md; docs/language/reference/dataset_types.md and docs/language/explanation/dataset_types.md; RFC 001 marked implemented with Design Decisions, Related links to incan#187 / InQL chore: Revisit InQL package/docs after Incan static capability gating (RFC 001) #10, and clarified split between library contract vs compiler enforcement.
• Risks: Stubs mean any code path that actually invokes dataset methods at runtime will hit NotImplementedError-style failures until execution is wired—acceptable for a contract-first 0.1.0 if docs and RFC say so.

Testing / verification

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

Manual verification notes:

• Optional: incan run / incan test on examples as you prefer; CI should already cover make ci.

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/rfcs/001_inql_dataset.md, docs/rfcs/README.md, docs/language/reference/dataset_types.md, docs/language/explanation/dataset_types.md, examples/README.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