feat: add block scoping for while/do/done#251
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Add scope_stack: List[List[str]] to TypeChecker and BytecodeCompiler. Push a new frame on WhileStart, pop it on WhileEnd. Variables first assigned inside a while block are tracked in the innermost scope frame. On WhileEnd those variables move to out_of_scope and any subsequent PushVar raises UndefinedName. Variables already in scope before the block (pre_scope_vars) or declared global are unaffected. Pattern bindings from `is` checks follow the same scoping rules. For-loop variables are covered automatically because for desugars to while. Add tests: while scoping, for scoping, outer mutation, sibling reuse, global bypass.
A variable first assigned inside a while block is marked out-of-scope on `done`. Reassigning the same name at the function (outer) scope did not clear that marking, so a later read was wrongly rejected as undefined — unlike sibling-while reuse, which did clear it. The pre-scope assignment path now clears the out-of-scope marking too. The scope-recording logic, previously duplicated between variable assignment and `is`-pattern bindings, is extracted into a single `record_var_scope` method so the fix lives in one place.
The bytecode compiler pushed and popped a scope_stack on while boundaries but never read it — scope enforcement lives entirely in the type checker.
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Summary
scope_stack: List[List[str]]toTypeCheckerandBytecodeCompilerWhileStart, pop onWhileEnd; variables first assigned inside a while block are block-local and inaccessible afterdonepre_scope_vars) andglobaldeclarations are unaffectedischecks follow the same rules;forloop variables covered automatically via desugar-to-whileTest plan
test_scope: while scoping, for scoping, outer mutation, sibling reuse, global bypassCloses #246