refactor: drop `_` prefix and `Interface` suffix from generated types

[pageton]

Summary

The code generator produces types with a leading _ prefix and an Interface suffix:

type _QueryInterface[T any] interface { ... }
type _QueryImpl[T any] struct { ... }

Two issues:

1. _ prefix — In Go, _ is the blank identifier. Using it as a name prefix is non-standard and confuses readers into thinking the variable/type is discarded.
2. Interface suffix — Per Go convention, interfaces are named after the behavior they describe (e.g. io.Reader, not io.ReaderInterface).

Proposed

type Query[T any] interface { ... }
type QueryImpl[T any] struct { ... }

Since generated code is in the same package as the constructor func Query[T any](...), the unexported interface name queryIface or similar would also work if name clashes are a concern.

Root Cause

• internal/gen/generator.go:611 — IfaceName: "_" + n.Name.Name
• internal/gen/template.go — uses {{$IfaceName}}Interface and {{$IfaceName}}Impl

Affected

All generated output files. The template in internal/gen/template.go and the naming logic in internal/gen/generator.go need updating.