fix(ts): instantiate recursive generic interface indexes at concrete type arguments

Related: #846, #950, #951. TypeScript 5.7.3 and current Home differ on a forward/recursive generic interface with inherited index signatures. Minimal strict/noEmit source:

interface Alpha<T> { make(): Beta<T> }
interface MixedBase<T> { readonly [key: string]: T | (() => Beta<T>); [index: number]: T }
interface Beta<T> extends Alpha<T>, MixedBase<T> {}
declare const alpha: Alpha<number>;
const beta = alpha.make();
beta["new"] = 2;
beta[0] = 2;
beta["0"] = 2;

TypeScript 5.7.3 reports only TS2542 at line 6, naming Beta<number>. Home reports TS2542 naming Beta<T> and an additional TS2322 at line 7 because it checks the mutable number index against unresolved T. The string-index write must fail and the number/numeric-string writes must remain legal. The generic-interface refresh and forward instantiation paths in packages/ts_checker/src/check.zig are candidates to inspect, but root cause has not yet been established. Fix the concrete substitution rather than suppressing TS2322; add checker/Program/CLI regressions and verify under the 3,840 MB guard. Keep #951 focused on readonly-domain provenance, with this issue tracking the separate concrete-generic instantiation failure.

The red, standalone reproducer is now on main in 23c23c7 at packages/ts_cli/testdata/issue952-forward-generic-index/. Rechecked with TypeScript 5.7.3 and a freshly built Home CLI: TypeScript gives one TS2542 at line 6 naming Beta<number>; Home gives TS2542 naming Beta<T> plus an invalid TS2322 at line 7. It is intentionally not in the passing stream suite until the concrete substitution is fixed.

Resolved by be95309 and d58b8f0 on main. The provisional callable return was refreshed in the formal generic scope; it now refreshes in the concrete receiver scope. The red fixture now matches TypeScript 5.7.3 exactly: one TS2542 naming Beta<number>, no false TS2322. Checker, Program, CLI stream, and tsc_main guarded suites exited 0; positive and negative controls are included.