refactor: extract string and array dispatch chains from generate()

[cs01]

Summary

• Extracts the 23-block string method dispatch chain from generate() into dispatchStringMethod() in new file string-dispatch.ts
• Extracts the 16-block array method dispatch chain into dispatchArrayMethod() in the same file
• Reduces consecutive if-return blocks in generate() from 39+ to 2, mitigating the native compiler's if-drop bug
• Preserves fall-through semantics: methods like concat, slice, includes that handle both string and array cases return null to allow the array dispatcher to handle them

Test plan

• All 433 tests pass (test:node)
• Self-hosting verification passes (verify:quick)
• Prettier formatting passes
• Pure refactor — no behavioral changes

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