Module emit 

AST walker that emits canonical Gruel text into a Printer.

Subsequent ADR-0093 phases extend the set of nodes handled here:

• Phase 1: smallest case (covered in lib.rs baseline).
• Phase 2: expressions, statements, patterns, types.
• Phase 3: top-level items (Function, StructDecl, EnumDecl, …).
• Phase 4: comment / blank-line weaving via a trivia table.

Exhaustive matches are deliberate: a new AST node kind triggers a compile-time error here rather than silently re-emitting nothing.

Functions

emit_ast

emit_block

emit_expr

emit_item

emit_pattern

emit_statement

emit_type_expr