feat(ir): add deterministic AST-to-IR lowering

[ofabiodev]

Summary

Adds the v0.1 intermediate representation and deterministic lowering from validated Puff programs into compiler-oriented IR.

Changes

• Add IR models for modules, namespaces, globals, functions, tags, commands, values, references, and structured text.
• Lower the v0.1 target program, including load/tick handlers, core.send, interpolation, parameters, and global identities.
• Add AST/IR goldens, boundary tests, deterministic ordering checks, and lowering regression coverage.

Testing

cd src
go test ./...
go test ./internal/lower -shuffle=on -count=50
go vet ./...
cd ..
make test
make check