feat(sema): add core semantic checks

[ofabiodev]

Summary

Adds semantic checking layer for Puff, validating scopes, names, visibility, required events, control flow, and basic type compatibility.

Changes

• Add semantic symbol resolution, type inference, global initialization, and import visibility checks
• Validate required load and tick events, return paths, stop, assignments, collections, and function arguments
• Add unit and end-to-end fixtures covering semantic diagnostics, scopes, cyclic globals, indexed paths, and type edge cases

Testing

make test
make build

cd src
go vet ./...
go test -count=1 ./...
go test -count=20 ./internal/sema