feat(keg): add omega calculation DSL

[jlrickert]

Summary

• add a parser and evaluator for the draft Omega DSL used by the .tapper/specs design
• support metadata and stat references, directional link sets, bindings, arithmetic, matching, weighted expressions, and aggregate functions
• expose a small environment interface so evaluation remains pure and independent of keg storage
• cover parsing, evaluation, link directionality, error handling, and the worked specification examples

Why

The current form-shaped maturity weight configuration became too complex as calculations grew. Omega DSL moves maturity calculation into one composable expression language while leaving structural schema declarations separate.

Scope

This is the foundation only. It does not yet wire omega: programs into SchemaDefinition or replace the current nested maturity weights. Publishing the draft specification and migrating schema calculation and replay behavior remain separate follow-up changes.

Verification

• go test ./...
• go vet ./pkg/keg/omegadsl
• gofmt -d pkg/keg/omegadsl