feat(parser): add statement expression parser

[ofabiodev]

Summary

Adds the parser implementation for Puff expressions, statements, blocks, and control flow.

Changes

• Add expression parsing with precedence, calls, variables, collections, ranges, and interpolation
• Add assignments, conditionals, loops, returns, stops, and effect statements
• Add diagnostic recovery, full grammar golden coverage, and malformed syntax tests

Testing

make test
make build

cd src
go vet ./...
go test ./internal/parser -cover