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/utils.rs

@@ -31,12 +31,26 @@ impl super::MirBuilder {
     /// Allocate a new ValueId in the appropriate context
     /// - Inside function: uses function-local allocator
     /// - Outside function: uses module-global allocator
+    ///
+    /// Phase 201-A: Skips reserved ValueIds (PHI dsts from LoopHeaderPhiBuilder)
+    /// to prevent carrier value corruption in JoinIR loops.
     #[inline]
     pub(crate) fn next_value_id(&mut self) -> super::ValueId {
-        if let Some(ref mut f) = self.current_function {
-            f.next_value_id() // Function context
-        } else {
-            self.value_gen.next() // Module context
+        loop {
+            let candidate = if let Some(ref mut f) = self.current_function {
+                f.next_value_id() // Function context
+            } else {
+                self.value_gen.next() // Module context
+            };
+
+            // Phase 201-A: Skip reserved PHI dst ValueIds
+            if !self.reserved_value_ids.contains(&candidate) {
+                return candidate;
+            }
+            // Reserved ID - try next one (loop continues)
+            if std::env::var("NYASH_201A_DEBUG").is_ok() {
+                eprintln!("[201-A] next_value_id: Skipping reserved {:?}", candidate);
+            }
         }
     }