Split emitter into focused components

[grian32]

Problem

emitter.go is currently a large switch that mixes expression lowering, statement lowering, control flow, type conversion, imports, builtins, sizing, and user-facing validation.

Tasks

• Split expression lowering from statement lowering.
• Split type conversion/sizing helpers.
• Split control-flow helpers.
• Split import declaration emission.
• Split builtin/std module hooks.
• Remove semantic checks after checker owns them.
• Add tests around behavior before/after refactor.

Ref: emitter/emitter.go.

Related: #4