feat(joinir): Phase 204 PHI Contract Verifier complete

Phase 204-3/5/6/7: PHI inputs verification, integration, testing, docs

Implementation:
- verify_phi_inputs_defined(): Conservative sanity checks (ValueId < 100000)
- Integration: All verifiers in verify_joinir_contracts()
- Testing: 821 tests PASS, no regressions

Task Status:
- ✅ 204-1: Design document (phase204-phi-contract-verifier.md)
- ✅ 204-2: PHI dst overwrite detection
- ✅ 204-3: PHI inputs sanity checks
- ⚠️ 204-4: JoinValueSpace region verification (deferred to Phase 205+)
  - Rationale: Requires LoopHeaderPhiInfo extension (4+ files)
  - Alternative: Conservative threshold checks in verify_phi_inputs_defined()
- ✅ 204-5: Integration (verify_joinir_contracts)
- ✅ 204-6: Tests (821 PASS)
- ✅ 204-7: Documentation (phase204 doc + CURRENT_TASK.md)

Verification Coverage:
- ✅ PHI exists (Phase 200-3)
- ✅ PHI dst match (Phase 200-3)
- ✅ PHI dst not overwritten (Phase 204-2) ✨
- ✅ PHI inputs sanity (Phase 204-3) ✨
- ⚠️ PHI inputs DFA (Phase 205+)
- ⚠️ ValueId regions (Phase 205+)

Design Principles:
- Debug-only (#[cfg(debug_assertions)])
- Fail-Fast (panic on violation)
- Zero cost in release builds

Files Modified:
- src/mir/builder/control_flow/joinir/merge/mod.rs (+115 lines)
- src/mir/builder/control_flow/joinir/merge/exit_line/reconnector.rs (1 line)
- docs/development/current/main/phase204-phi-contract-verifier.md (updated)
- CURRENT_TASK.md (Phase 204 complete)

Success Criteria: 4/5 met (1 deferred with rationale)

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

@@ -889,6 +889,54 @@ fn get_instruction_dst(instr: &crate::mir::MirInstruction) -> Option<crate::mir:
     }
 }
 
+/// Verify PHI inputs are defined (Phase 204-3 - Conservative sanity checks)
+///
+/// # Checks
+///
+/// 1. PHI inputs have reasonable ValueId values (< threshold)
+/// 2. No obviously undefined values (e.g., suspiciously large IDs)
+///
+/// # Note
+///
+/// Full data-flow analysis (DFA) verification is deferred to Phase 205+.
+/// This function only performs conservative sanity checks.
+///
+/// # Panics
+///
+/// Panics in debug mode if suspicious PHI inputs are detected.
+#[cfg(debug_assertions)]
+fn verify_phi_inputs_defined(
+    func: &crate::mir::MirFunction,
+    header_block: crate::mir::BasicBlockId,
+) {
+    let header_block_data = func.blocks.get(&header_block).unwrap_or_else(|| {
+        panic!(
+            "[JoinIRVerifier] Header block {} not found ({} blocks in func)",
+            header_block,
+            func.blocks.len()
+        )
+    });
+
+    for instr in &header_block_data.instructions {
+        if let crate::mir::MirInstruction::Phi { dst, inputs, type_hint: _ } = instr {
+            for (value_id, pred_block) in inputs {
+                // Conservative sanity check: ValueId should not be suspiciously large
+                // Phase 201 JoinValueSpace uses regions:
+                // - PHI Reserved: 0-99
+                // - Param: 100-999
+                // - Local: 1000+
+                // - Reasonable max: 100000 (arbitrary but catches obvious bugs)
+                if value_id.0 >= 100000 {
+                    panic!(
+                        "[JoinIRVerifier/Phase204-3] PHI {:?} has suspiciously large input {:?} from predecessor block {:?}",
+                        dst, value_id, pred_block
+                    );
+                }
+            }
+        }
+    }
+}
+
 /// Verify all loop contracts for a merged JoinIR function
 ///
 /// This is the main entry point for verification. It runs all checks
@@ -907,5 +955,6 @@ fn verify_joinir_contracts(
 ) {
     verify_loop_header_phis(func, header_block, loop_info, boundary);
     verify_no_phi_dst_overwrite(func, header_block, loop_info); // Phase 204-2
+    verify_phi_inputs_defined(func, header_block); // Phase 204-3
     verify_exit_line(func, exit_block, boundary);
 }