fix(phase161): Use ArrayBox.get() instead of at() for VM compatibility

- Replace .at() with .get() in mir_analyzer.hako (10 occurrences)
- Fix test_rep1_inline.hako and test_mir_analyzer.hako
- Builtin ArrayBox only supports .get(), not .at()

Phase 161-2 MIR JSON parsing tests now pass:
- JSON object parsing: PASS
- functions array extraction: PASS
- Function name extraction: PASS
- blocks extraction: PASS
- PHI instruction detection: PASS (found PHI at block=10 dst=r30)

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

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

local_tests/phase161/test_mir_analyzer.hako

@@ -12,11 +12,10 @@ box TestRunner {
     print("Test: " + name)
     print("==================================================")
 
-    // Parse MIR JSON
-    me.analyzer = new MirAnalyzerBox()
-    local result = me.analyzer.birth(mir_json_text)
+    // Parse MIR JSON - use constructor directly
+    me.analyzer = new MirAnalyzerBox(mir_json_text)
 
-    if result == null {
+    if me.analyzer == null {
       print("FAIL: Failed to parse MIR JSON")
       return null
     }

local_tests/phase161/test_rep1_inline.hako

@@ -113,7 +113,7 @@ static box Main {
       print("  [OK] Correct PHI count (1)")
       pass_count = pass_count + 1
 
-      local phi = phi_list.at(0)
+      local phi = phi_list.get(0)
       local block_id = phi.get("block_id")
       local dest = phi.get("dest")
       local incoming_count = phi.get("incoming_count")