Fix boolean initializer diagnostic location

[Devon7925]

Summary

• capture the initializer start position when parsing locals so diagnostics can use it
• fall back to the initializer start when a boolean local reports a type mismatch without an expression location
• assert the reported location in the boolean locals regression test

Testing

• bun test test/booleans.test.ts --filter "boolean locals reject integer initializers"

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https://chatgpt.com/codex/tasks/task_e_68fbd830b2b8832993d20cc1955fa4e7