flat codegen: an enum value bound from a call declines emit; blocks runnable cross-module enums

Symptom

A payload-free (C-like) enum value returned from a call and bound to a local declines flat emit — even with no match:

enum Color { Red, Green, Blue }
fn pick() -> Color { return Color::Green; }
fn main() -> i32 { let c = pick(); return 0; }   // [flat-dbg] emit declined for fn main

VX_FLAT_DBG=1: emit declined for fn main → a function outside the emitter subset. Intra-module this is harmless (the AST codegen path handles it), but it blocks the cross-module case: an imported enum can only link on the flat path (the consumer has no AST for the library body), so a flat decline there is a hard error.

By contrast, a local enum construct + match does run:

fn main() -> i32 { let c = Color::Green; match c { Color::Red => {return 1;} Color::Green => {return 2;} Color::Blue => {return 3;} } }  // -> 2

Why

The flat emitter already models a payload-free enum as a bare i32 discriminant (EmitCtx.enums, built from ImmutableGlobalRegistry.enum_variants, src/codegen/flat.rs). But binding an enum value produced by a call (an enum-typed return threaded through a let) falls outside the emitter subset and declines. That is the gap to close.

Scope for runnable cross-module enums

Two pieces, in order:

1. Flat emitter: emit an enum-typed value bound from a call (and the match over an imported enum) instead of declining — the substantive part, a flat-codegen coverage gap in the class of hiraditya/Vx.1#273.
2. Codec: serialize enum_variants into the .vxlib (it is populated at freeze but set empty on deserialize in src/metadata.rs; merge_from already carries it). Necessary so EmitCtx.enums is populated for an imported enum — but useless until (1) lands, so it should ride with (1), not alone.

Context

Found while extending the hiraditya/Vx.1#219 cross-module surface: functions and structs work end-to-end from a .vxlib (construct, field-access, param, return — all runnable), because the flat emitter handles them. Enums do not, because of (1). Data-carrying (tagged-union) enums are the separate, blocked hiraditya/Vx.1#233; this issue is only the payload-free case. Degrades gracefully today (frontend type-checks the imported enum; codegen gives a clean error, no ICE).

Start in src/codegen/flat.rs, tracing EmitCtx.enums and ImmutableGlobalRegistry.enum_variants, and reproduce the payload-free enum call with VX_FLAT_DBG=1. Then inspect src/metadata.rs and its deserialize path. Done means call-bound enum values and matches over imported enums emit successfully, with enum_variants preserved in .vxlib metadata.