feat: add execution flow parser

[mattiamanzati]

Summary

• add a new execution-flow parser that models value flow more directly than the current piping-flow parser, including data-first, data-last, pipe, pipeable, and function-pipe relationships
• add Mermaid flow baselines for Effect fixtures so execution graphs can be inspected and evolved alongside diagnostics
• refactor pipe-call type extraction into ParsePipeCall and extend the internal graph helpers needed by the new flow representation

Why

The new execution-flow engine preserves richer information about how values move through code, which makes it a better foundation for future diagnostics than the existing linear piping-flow parser. It is intended to gradually replace piping-flow parsing as diagnostics move to the more expressive graph model.

Validation

• pnpm setup-repo
• pnpm lint
• pnpm check
• pnpm test