Goals

Add an opt-in faster streaming backend for JSONLoader while preserving the current default parser and atomic JSON.parse behavior.

Changes

• Added json.backend: 'clarinet' | 'fast' to JSONLoaderOptions, defaulting to 'clarinet'.
• Wired json.backend: 'fast' through JSONLoader.parseInBatches() to use the fast streaming parser backend.
• Set _FastJSONLoader to use the same json.backend: 'fast' option path.
• Updated streaming JSON tests to cover both default and fast backends through JSONLoader.
• Added TSDoc for the new backend option and parser-backend helper paths.
• Documented the fast backend in JSON module docs, JSONLoader API docs, and whats-new.