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/decls.rs

@@ -116,8 +116,16 @@ impl super::MirBuilder {
                         reg.ensure_slot(p, ty);
                     }
                 }
+                // Phase 200-C: Store fn_body_ast for inline main() lowering
+                eprintln!("[build_static_main_box] Storing fn_body_ast with {} nodes for inline main()", body.len());
+                self.fn_body_ast = Some(body.clone());
+
                 // Lower statements in order to preserve def→use
                 let lowered = self.cf_block(body.clone());
+
+                // Phase 200-C: Clear fn_body_ast after main() lowering
+                self.fn_body_ast = None;
+
                 self.variable_map = saved_var_map;
                 lowered
             } else {