fix: split().join() chaining produces correct string output

[cs01]

Summary

• "hello world".split(" ").join("-") was producing garbage numeric output instead of "hello-world"
• Root cause: generateSplit never called setVariableType on its result temp, so downstream join() couldn't detect it was a %StringArray* and defaulted to numeric array join
• One-line fix: add ctx.setVariableType(result, "%StringArray*") after the phi node in split.ts

Test plan

• New fixture strings/string-split-chain.ts — chained split+join on string literals
• All 618 tests pass
• Self-hosting verification passes

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