feature - implement RFC 022 hashing functions (#39)

[dannymeijer]

Summary

This PR implements the first RFC 022 format-function slice: deterministic string hashing helpers. It adds registry-backed md5, sha224, sha256, sha384, sha512, and sha2 helpers, records stable Substrait extension anchors for the concrete helpers, and keeps sha2(...) as a compatibility rewrite over supported literal bit lengths. RFC 022 remains In Progress because JSON, CSV, URL, and dynamic-value helper semantics are still explicitly future scope.

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: Authors can build lowercase hexadecimal hash expressions with md5(...), concrete SHA-2 helpers, or sha2(expr, 224|256|384|512).
• Internals: Concrete hash helpers declare registry-owned Substrait extension mappings. sha2(...) is a compatibility helper that rewrites to the matching concrete SHA-2 helper and rejects unsupported digest lengths before backend lowering.
• Risks: This PR intentionally does not implement JSON/CSV parsers, URL helpers, dynamic-value predicates, or binary hash-output variants. RFC 022 documents that remaining scope.

Testing / verification

• make fmt
• incan test tests/test_hashing_functions.incn
• incan test tests/test_function_registry.incn
• incan test tests/test_substrait_plan.incn
• incan test tests/test_session_projection.incn
• 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:

• make test and make pre-commit both passed with 146 tests.
• make registry-metadata passed with 92 helpers.
• incan.lock is out of date warnings appeared as expected; this PR does not refresh the lock.

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/format.md, docs/language/reference/functions/index.md, docs/release_notes/v0_1.md, docs/rfcs/022_semi_structured_format_functions.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

Stack order: merge after #48. This PR is partial RFC 022 scope and should not close the RFC issue yet.

Refs #39