obs(joinir): Phase 72 - PHI Reserved Region Observation Complete

## Summary
Observed PHI dst ValueId distribution to determine if verifier can enforce
reserved region (0-99). **Conclusion: Verifier strengthening NOT recommended.**

## Key Finding
PHI dst allocation does NOT architecturally respect reserved region:
- PHI dst comes from `builder.next_value_id()` (host MirBuilder)
- Reserved region (0-99) is a JoinValueSpace contract for JoinIR lowering
- These are separate allocation pools with no enforcement mechanism
- Current stability is accidental (ValueId allocation ordering)

## Evidence
Manual verification (`apps/tests/loop_min_while.hako`):
- PHI dst = %3 (ValueId(3)) ✅ in reserved region
- Works because PHI allocated early in function (0-20 typical)
- JoinValueSpace allocates high (100+, 1000+)
- Accidental separation, not enforced

## Implementation
Added observation infrastructure (debug-only):
- `src/mir/join_ir/verify_phi_reserved.rs` (266 lines)
  - PHI dst observer with distribution analyzer
  - Region classifier (Reserved/Param/Local)
  - Human-readable report generator
- Instrumentation at PHI allocation points:
  - loop_header_phi_builder.rs:94, 151
  - Debug-only `observe_phi_dst()` calls

## Test Results
- Unit tests: ✅ 4/4 PASS (verify_phi_reserved module)
- Lib tests: ✅ 950/950 PASS, 56 ignored
- Normalized tests: ✅ 54/54 PASS
- Manual verification: ✅ PHI dst in 0-99 observed

## Decision: Document, Don't Enforce
**Rationale**:
1. No architectural mechanism to enforce PHI dst ∈ [0, 99]
2. Adding verifier creates false assumptions about allocation order
3. Current system stable through separate pools (950/950 tests)
4. Future architectural fix possible (Phase 73+) but not urgent

## Documentation
- PHASE_72_SUMMARY.md: Executive summary and implementation record
- phase72-phi-reserved-observation.md: Detailed findings and analysis
- CURRENT_TASK.md: Phase 72 completion entry

## Next Steps
- Phase 73: Update architecture overview with Phase 72 findings
- Optional: Explicit PHI reserved pool (future enhancement)

🤖 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/merge/loop_header_phi_builder.rs

@@ -88,6 +88,11 @@ impl LoopHeaderPhiBuilder {
 
         // Allocate PHI for loop variable
         let loop_var_phi_dst = builder.next_value_id();
+
+        // Phase 72: Observe PHI dst allocation
+        #[cfg(debug_assertions)]
+        crate::mir::join_ir::verify_phi_reserved::observe_phi_dst(loop_var_phi_dst);
+
         info.carrier_phis.insert(
             loop_var_name.to_string(),
             CarrierPhiEntry {
@@ -145,6 +150,11 @@ impl LoopHeaderPhiBuilder {
             };
 
             let phi_dst = builder.next_value_id();
+
+            // Phase 72: Observe PHI dst allocation
+            #[cfg(debug_assertions)]
+            crate::mir::join_ir::verify_phi_reserved::observe_phi_dst(phi_dst);
+
             info.carrier_phis.insert(
                 name.clone(),
                 CarrierPhiEntry {