Feature: validate typed scalar expression inputs (#57)

[dannymeijer]

Summary

This PR implements InQL issue #57 by making scalar helper contracts typed, schema-aware, and source-driven. Public helpers now declare precise ValueOrColumn inputs and typed expression results where InQL knows the contract; Prism carries row-schema facts forward; Substrait lowering rejects schema-incompatible column references before backend execution. It also updates the cast family to use Incan primitive type-token targets such as cast(col("amount_text"), float) and try_cast(col("status"), int) while keeping explicit string targets for compatibility spellings.

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: Helpers now accept natural primitive values or typed column expressions at typed boundaries. For example, gt(col("qty"), 2), substring(col("sku"), 1, 3), make_date(2026, 5, 30), and cast(col("amount_text"), float) are the intended shapes. Broad lit(...) remains a structural escape hatch, not a way to bypass typed helper contracts.
• Source contract cleanup: Removes registry-side scalar argument/result metadata and derives scalar contracts from checked public helper signatures instead. The registry still owns identity, lifecycle, policy, alias, aggregate/window/generator, and Substrait mapping metadata.
• Typed expression model: Adds typed scalar wrappers such as boolean, integer, number, string, timestamp, and temporal column expressions. Helpers return concrete typed wrappers where their result is known.
• Schema validation: Prism/Substrait lowering validates typed helper inputs against the current query schema before the backend adapter sees the plan. Backend inability remains backend-specific; type-contract violations are InQL planning diagnostics.
• Cast DX: cast and try_cast now support primitive type-token overloads for bool, int, float, and str; safe_cast is an Incan alias of try_cast plus registry compatibility alias metadata. String target spellings remain for compatibility aliases such as int64 and float64.
• Function coverage: Tightens contracts across operators, predicates, math, strings, regex, encoding, hashing, datetime, JSON/CSV/URL format helpers, nested data helpers, aggregate/window inputs, and examples/tests.
• Incan dependency: This PR currently depends on dannys-code-corner/incan#751 / 0.3.0-rc47. CI is temporarily pinned to feature/750-primitive-type-tokens; after #751 lands on release/v0.3, this should be switched back to release/v0.3 with the same expected rc or final 0.3 version.
• Review finding record: Source-quality findings were recorded under .agents/state/review-report.incan-source-quality.md locally and snapshotted in /tmp/incan-review-findings/2026-06-03-branch-feature-57-typed-scalar-validation-incan-source-quality-followup.md.

Testing / verification

• make fmt INCAN=<rc47 compiler> -> 0 files changed
• make fmt-check INCAN=<rc47 compiler> -> 276 src, 20 tests, 8 examples already formatted
• make test-style -> passed
• make registry-metadata INCAN=<rc47 compiler> -> 199 helpers
• make build INCAN=<rc47 compiler> -> passed
• make test INCAN=<rc47 compiler> -> 216 passed
• make build-locked INCAN=<rc47 compiler> -> passed
• make test-locked INCAN=<rc47 compiler> -> 216 passed
• make smoke-consumer INCAN=<rc47 compiler> -> passed after allowing Cargo network access
• git diff --check

Focused verification during the review/fix loop:

• incan test tests/test_session_filters.incn -> 6 passed
• incan test tests/test_session_projection.incn -> 17 passed
• incan test tests/test_substrait_plan.incn -> 50 passed
• incan test tests/test_function_registry.incn -> 19 passed
• incan test tests/test_core_scalar_functions.incn -> 5 passed
• incan test tests/test_common_scalar_functions.incn -> 5 passed
• incan test tests/test_nested_data_functions.incn -> 3 passed

Docs impact

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

If docs updated:

• Links: README.md, docs/language/reference/functions/index.md, builder/reference docs, nested function reference docs, and 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

Closes #57