feature - implement core scalar functions and operators (#32)

[dannymeijer]

Summary

This PR implements InQL RFC 015 / issue #32 by moving scalar helpers onto a registry-backed ColumnExpr application model, adding the core scalar/operator helper slice, and closing RFC 015 as Implemented. It also removes DataFusion-specific semantics from the function/lowering path by keeping Substrait as the IR boundary and routing Session execution through a backend adapter dispatch layer.

Type of change

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

Area(s)

Select the primary areas touched (labels sync from checked lines when the triage workflow runs):

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

Key details

• User-facing behavior: Adds the RFC 015 core scalar surface: casts, comparisons, boolean logic, null/NaN predicates, arithmetic, conditionals, membership/range predicates, and ordering helpers. Ordering helpers are valid through order_by([...]) and lower to real Substrait sort fields.
• Internals: Replaces bespoke scalar function/operator expression variants with registry-backed ScalarFunctionApplicationExpr, keeps structural column/literal nodes, splits helper modules by logical family, and extends Substrait lowering through registry metadata, built-in Rex shapes, and structural sort-field lowering. Session execution now dispatches through a backend adapter boundary over Substrait plans; DataFusion remains the first adapter.
• Risks: Broad public helper and lowering surface. Existing helper compatibility is covered, but downstream callers may notice that scalar function metadata and order_by now use the registry-backed/explicit-sort-key model.

Testing / verification

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

Manual verification notes:

• make fmt-check
• make test-style
• make registry-metadata (passes; still emits the known stale-lock warning)
• make build
• make build-locked
• make test (108 passed)
• make smoke-consumer
• git diff --check

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/index.md, docs/language/reference/dataset_methods.md, docs/language/reference/execution_context.md, docs/release_notes/v0_1.md, docs/rfcs/015_core_scalar_functions.md, docs/rfcs/014_function_registry.md, docs/rfcs/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

Closes #32