feat(parser): add top level parser

[ofabiodev]

Summary

Adds the Puff v0.1 top-level parser for metadata, imports, functions, events, and global assignments.

Changes

• Parse requirements, function signatures, nested types, events, globals, and public declarations.
• Add source spans, contextual identifiers, block balancing, and parser recovery.
• Add golden AST coverage and diagnostics for invalid top-level syntax, missing tokens, unexpected tokens, and missing end.

Testing

make test
make build
cd src
go vet ./...