feat(joinir): Phase 200-B/C/D capture analysis + Phase 201-A reserved_value_ids infra

Phase 200-B: FunctionScopeCaptureAnalyzer implementation
- analyze_captured_vars_v2() with structural loop matching
- CapturedEnv for immutable function-scope variables
- ParamRole::Condition for condition-only variables

Phase 200-C: ConditionEnvBuilder extension
- build_with_captures() integrates CapturedEnv into ConditionEnv
- fn_body propagation through LoopPatternContext to Pattern 2

Phase 200-D: E2E verification
- capture detection working for base, limit, n etc.
- Test files: phase200d_capture_minimal.hako, phase200d_capture_in_condition.hako

Phase 201-A: MirBuilder reserved_value_ids infrastructure
- reserved_value_ids: HashSet<ValueId> field in MirBuilder
- next_value_id() skips reserved IDs
- merge/mod.rs sets/clears reserved IDs around JoinIR merge

Phase 201: JoinValueSpace design document
- Param/Local/PHI disjoint regions design
- API: alloc_param(), alloc_local(), reserve_phi()
- Migration plan for Pattern 1-4 lowerers

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

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

@@ -54,6 +54,10 @@ pub struct LoopPatternContext<'a> {
 
     /// Phase 192: Pattern classification based on features
     pub pattern_kind: LoopPatternKind,
+
+    /// Phase 200-C: Optional function body AST for capture analysis
+    /// None if not available, Some(&[ASTNode]) if function body is accessible
+    pub fn_body: Option<&'a [ASTNode]>,
 }
 
 impl<'a> LoopPatternContext<'a> {
@@ -87,8 +91,22 @@ impl<'a> LoopPatternContext<'a> {
             has_break,
             features,
             pattern_kind,
+            fn_body: None, // Phase 200-C: Default to None
         }
     }
+
+    /// Phase 200-C: Create context with fn_body for capture analysis
+    pub fn with_fn_body(
+        condition: &'a ASTNode,
+        body: &'a [ASTNode],
+        func_name: &'a str,
+        debug: bool,
+        fn_body: &'a [ASTNode],
+    ) -> Self {
+        let mut ctx = Self::new(condition, body, func_name, debug);
+        ctx.fn_body = Some(fn_body);
+        ctx
+    }
 }
 
 /// Phase 193: Feature extraction moved to ast_feature_extractor module