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
+    // Parse MIR JSON - use constructor directly
-    me.analyzer = new MirAnalyzerBox()
+    me.analyzer = new MirAnalyzerBox(mir_json_text)
-    local result = me.analyzer.birth(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")

tools/hako_shared/mir_analyzer.hako

@@ -106,7 +106,7 @@ box MirAnalyzerBox {
 
     local i = 0
     loop(i < blocks.size()) {
-      local block = blocks.at(i)
+      local block = blocks.get(i)
       local instructions = block.get("instructions")
 
       if instructions != null {
@@ -115,7 +115,7 @@ box MirAnalyzerBox {
         // Check for PHI and Branch instructions
         local j = 0
         loop(j < instructions.size()) {
-          local inst = instructions.at(j)
+          local inst = instructions.get(j)
           local op = inst.get("op")
 
           if op == "phi" {
@@ -171,14 +171,14 @@ box MirAnalyzerBox {
 
     local i = 0
     loop(i < blocks.size()) {
-      local block = blocks.at(i)
+      local block = blocks.get(i)
       local block_id = block.get("id")
       local instructions = block.get("instructions")
 
       if instructions != null {
         local j = 0
         loop(j < instructions.size()) {
-          local inst = instructions.at(j)
+          local inst = instructions.get(j)
           local op = inst.get("op")
 
           if op == "phi" {
@@ -230,7 +230,7 @@ box MirAnalyzerBox {
     local block_map = new MapBox()
     local i = 0
     loop(i < blocks.size()) {
-      local block = blocks.at(i)
+      local block = blocks.get(i)
       local block_id = block.get("id")
       block_map.set("" + block_id, i)
       i = i + 1
@@ -241,7 +241,7 @@ box MirAnalyzerBox {
 
     i = 0
     loop(i < blocks.size()) {
-      local block = blocks.at(i)
+      local block = blocks.get(i)
       local block_id = block.get("id")
       local instructions = block.get("instructions")
 
@@ -249,7 +249,7 @@ box MirAnalyzerBox {
         // Check last instruction for control flow
         local last_idx = instructions.size() - 1
         if last_idx >= 0 {
-          local last_inst = instructions.at(last_idx)
+          local last_inst = instructions.get(last_idx)
           local op = last_inst.get("op")
 
           // Check jump instruction
@@ -316,7 +316,7 @@ box MirAnalyzerBox {
     if funcIndex < 0 { return null }
     if funcIndex >= me._functions.size() { return null }
 
-    return me._functions.at(funcIndex)
+    return me._functions.get(funcIndex)
   }
 
   // Internal: Check if function has backward edges (loop indicator)
@@ -325,14 +325,14 @@ box MirAnalyzerBox {
 
     local i = 0
     loop(i < blocks.size()) {
-      local block = blocks.at(i)
+      local block = blocks.get(i)
       local block_id = block.get("id")
       local instructions = block.get("instructions")
 
       if instructions != null {
         local last_idx = instructions.size() - 1
         if last_idx >= 0 {
-          local last_inst = instructions.at(last_idx)
+          local last_inst = instructions.get(last_idx)
           local op = last_inst.get("op")
 
           if op == "jump" {