feat(joinir): Phase 170-D-impl-2 Minimal analysis logic with condition_var_analyzer

Implement variable extraction and scope classification with Box separation:

New module:
- src/mir/loop_pattern_detection/condition_var_analyzer.rs (270 lines)

Public API (pure functions):
- extract_all_variables(): Recursive AST traversal for variable collection
  * Handles: Variable, UnaryOp, BinaryOp, MethodCall, FieldAccess, Index, If
  * Deduplicates via HashSet automatically
  * Returns all variable names found in expression

- is_outer_scope_variable(): Scope classification heuristic
  * Phase 170-D simplified: checks pinned set and header-only definitions
  * Conservative: defaults to LoopBodyLocal if uncertain
  * Returns true only for definitely outer-scope variables

Integration (LoopConditionScopeBox.analyze()):
- Delegated to condition_var_analyzer functions
- Maintains original 3-level classification (LoopParam / OuterLocal / LoopBodyLocal)
- Cleaner separation: analyzer = pure logic, Box = orchestration

Test coverage:
- 12 unit tests in condition_var_analyzer
  * Variable extraction: single, multiple, nested, deduped
  * Unary/Binary operations
  * Literal handling
  * Scope classification with mocked LoopScopeShape
  * Pinned variable detection
  * Header-only and multi-block definitions

Architecture improvements:
- Pure functions enable independent testing and reuse
- Fail-Fast principle: conservative scope classification
- Phase 170-D design: simple heuristics sufficient for initial detection

Build: ✅ Passed with no errors

🤖 Generated with Claude Code
Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

src/mir/loop_pattern_detection/mod.rs

@@ -752,5 +752,6 @@ mod tests {
     }
 }
 
-// Phase 170-D: Loop Condition Scope Analysis Box
+// Phase 170-D: Loop Condition Scope Analysis Boxes
 pub mod loop_condition_scope;
+pub mod condition_var_analyzer;