BridgeJS: Fix name collision for same-named nested structs

[krodak]

Overview

Follow-up to #735. When two different parent types both have a nested struct with the same short name (e.g., User.Stats and Player.Stats), the generated JavaScript produces duplicate const declarations and overwrites the same structHelpers.Stats key, causing a SyntaxError at parse time.

1. JS struct helpers use abiName (underscore-separated) for unique internal keys. structHelpers.User_Stats and structHelpers.Player_Stats instead of both being structHelpers.Stats. This covers helper factories, registration, and all structHelpers.<name>.lower/lift() call sites in both BridgeJSLink.swift and JSGlueGen.swift.

2. TypeScript declarations use TS namespaces for nested struct interfaces. Instead of export interface User_Stats, the .d.ts now emits:

export namespace User {
    export interface Stats { ... }
}

This matches the existing namespace pattern used for enums and keeps the public API hierarchical. The NamespaceBuilder was extended to handle structs alongside enums/classes. Struct-only namespace nodes are excluded from the Exports type to avoid duplicate keys.

3. tsType for .swiftStruct returns the dotted name directly (e.g., User.Stats), which TypeScript resolves through the namespace. Non-nested structs are unaffected.

4. E2e runtime test added. NestedStructGroupA.Metadata and NestedStructGroupB.Metadata (two same-named structs under different namespace enums) are roundtripped through JS to verify no data corruption from helper key collision.