The AST enum stays flat

The surface AST is a single flat enum, and it stays that way even as it passes twenty variants. Grouping the variants into nested sub-enums “for tidiness” is not an improvement here; it would:

• add an indirection layer that every match must thread through;
• obscure the one-to-one correspondence between grammar productions and variants;
• buy nothing the flat form lacks.

This is a deliberate standing choice, recorded so it is not “cleaned up” by a future pass. Do not group the AST variants.

(Recorded 2026-05-30; the decision itself predates the wiki.)

See also core, ir-pure-cbpv.