Implement RFC 018 common scalar catalog

[dannymeijer]

Summary

Completes the remaining RFC 018 common scalar catalog work after the earlier math slice. This adds registry-backed math, string, text encoding, regex, date/time, and compatibility alias helpers, keeps Substrait mappings registry-owned, and adds DataFusion adapter UDFs only where the first backend does not expose a direct API-level equivalent for the InQL contract.

Follow-up review polish in the latest commit keeps concat(...) and concat_ws(...) as variadic helper surfaces, removes redundant DataFusion adapter UDF wrapper code, and tightens regex docs so the portable contract is Rust-regex-compatible rather than DataFusion-owned.

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 now use the full RFC 018 scalar catalog: remaining math helpers, common string helpers, text encoding helpers, regex helpers, date/time helpers, and compatibility aliases such as substr, ucase, lcase, dateadd, datediff, and safe_cast.
• Internals: helpers register through the existing decorator-driven function registry, declare Substrait extension mappings, and execute through DataFusion APIs/UDFs without making DataFusion the semantic owner. RFC 013 is moved to In Progress and RFC 014/015/018 metadata is cleaned up.
• Risks: date/time and regex behavior is intentionally documented to the portable contract; broader semi-structured/binary logical values remain owned by RFC 026, not hidden in RFC 018.

Testing / verification

• make fmt
• make fmt-check
• make test-style
• make registry-metadata
• make build
• make test
• make smoke-consumer
• make pre-commit
• Manual verification described below

Manual verification notes:

• Ran all successful commands with /Users/danny/Development/encero/incan/target/debug/incan from Incan release/v0.3 / 0.3.0-rc30.
• Focused checks passed: incan test tests/test_common_scalar_functions.incn, incan test tests/test_function_registry.incn, incan test tests/test_substrait_plan.incn, and incan test tests/test_session_projection.incn.
• make test passed: 194 tests.
• make smoke-consumer INCAN=/Users/danny/Development/encero/incan/target/debug/incan passed with network access for crates.io resolution.
• make pre-commit passed, including format, test-style, registry metadata, build, and the 194-test suite.
• git diff --check passed.

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/018_common_scalar_function_catalog.md, docs/rfcs/013_function_catalog_program.md, docs/language/reference/functions/index.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

Closes #35.