Emitter crash `array of set (frontend rejects these)` / `array of dyn (frontend rejects these)` on `Array.from({length}, () => new Set())`

[xXCrash2BomberXx]

Array.from({ length: N }, () => new Set()) crashes the backend emitter with an uncaught exception rather than a diagnostic. The thrown message ("frontend rejects these") says the frontend is supposed to reject this construct before it reaches codegen — so this is the frontend's rejection guard failing to fire, not a feature request for array-of-Set support.

Minimal Demo

const arr = Array.from({ length: 2 }, () => new Set());
console.log(arr.length);

Output

file://[...]/moon/node_modules/@scriptc/compiler/dist/backend/emission/emit-types.js:531
            throw new Error(`emitter bug: array of ${elem.kind} (frontend rejects these)`);
                  ^

Error: emitter bug: array of dyn (frontend rejects these)
    at elemKindC (file://[...]/node_modules/@scriptc/compiler/dist/backend/emission/emit-types.js:531:19)
    at arrNewC (file://[...]/node_modules/@scriptc/compiler/dist/backend/emission/emit-shapes.js:528:31)
    at CEmitter.arrNewC (file://[...]/node_modules/@scriptc/compiler/dist/backend/emission/emitter.js:1230:16)
    at emitExpr (file://[...]/node_modules/@scriptc/compiler/dist/backend/emission/emit-exprs.js:1114:45)
    at CEmitter.emitExpr (file://[...]/node_modules/@scriptc/compiler/dist/backend/emission/emitter.js:1834:16)
    at emitStmt (file://[...]/node_modules/@scriptc/compiler/dist/backend/emission/emit-stmts.js:189:25)
    at CEmitter.emitStmt (file://[...]/node_modules/@scriptc/compiler/dist/backend/emission/emitter.js:1717:16)
    at emitStmts (file://[...]/node_modules/@scriptc/compiler/dist/backend/emission/emit-stmts.js:139:11)
    at CEmitter.emitStmts (file://[...]/node_modules/@scriptc/compiler/dist/backend/emission/emitter.js:1714:16)
    at emitFunction (file://[...]/node_modules/@scriptc/compiler/dist/backend/emission/emit-stmts.js:64:7)