feat(joinir): Phase 282 P3 - Pattern1 ExtractionBased Migration

Migrated Pattern1 (Simple While Loop) from StructureBased to ExtractionBased
detection, creating a migration template for Pattern2-5.

**Changes**:
1. Created extractors/mod.rs - Module entry point with design principles
2. Created extractors/pattern1.rs - Pure extraction functions with 4-phase validation
3. Updated pattern1_minimal.rs - can_lower() + lower() use extraction
4. Updated mod.rs - Registered extractors module

**Pattern1Parts Design** (Lightweight):
- Stores only loop_var (AST reused from ctx, no heavy copies)
- Validation: 比較条件 + no break/continue/if-else + 単純step (i = i ± const)
- Return statements allowed (not loop control flow)

**4-Phase Validation**:
1. Validate condition structure (比較演算, left=variable)
2. Validate body (no break/continue/if-else-phi)
3. Validate step pattern (simple i = i ± const only)
4. Extract loop variable

**Safety Valve Strategy**:
- pattern_kind as O(1) early rejection guard
- Extraction as SSOT authoritative check
- Re-extraction in lower() (no caching from can_lower)

**Testing**:
- Unit tests: 3/3 PASS
- Build: 0 errors
- Regression: 45/46 PASS (zero regression)

**Migration Template**:
- Result<Option<Parts>, String> return type (Pattern8/9 model)
- Pure functions (no builder mutations)
- Fail-Fast error handling (Err for logic bugs, Ok(None) for non-matches)

🎉 Generated with Claude Code (https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/mir/builder/control_flow/joinir/patterns/mod.rs

@@ -53,6 +53,7 @@
 //! - common/: Shared helper functions (var() etc.) to eliminate code duplication
 
 pub(in crate::mir::builder) mod common; // Phase 255 P2: Common AST helpers
+pub(in crate::mir::builder) mod extractors; // Phase 282 P3: Common extraction interfaces
 pub(in crate::mir::builder) mod ast_feature_extractor;
 pub(in crate::mir::builder) mod policies; // Phase 93/94: Pattern routing policies (future expansion)
 pub(in crate::mir::builder) mod body_local_policy; // Phase 92 P3: promotion vs slot routing