feat(joinir): Phase 185-187 body-local infrastructure + string design

Phase 185: Body-local Pattern2/4 integration skeleton
- Added collect_body_local_variables() helper
- Integrated UpdateEnv usage in loop_with_break_minimal
- Test files created (blocked by init lowering)

Phase 186: Body-local init lowering infrastructure
- Created LoopBodyLocalInitLowerer box (378 lines)
- Supports BinOp (+/-/*//) + Const + Variable
- Fail-Fast for method calls/string operations
- 3 unit tests passing

Phase 187: String UpdateLowering design (doc-only)
- Defined UpdateKind whitelist (6 categories)
- StringAppendChar/Literal patterns identified
- 3-layer architecture documented
- No code changes

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

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

apps/tests/phase185_p2_body_local_int_min.hako

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+// Phase 185: Minimal JsonParser-style loop with body-local integer calculation
+// Tests Pattern2 integration with LoopBodyLocalEnv
+
+static box Main {
+    main() {
+        local sum = 0
+        local pos = 0
+        local start = 0
+        local end = 5
+
+        // Pattern2: break loop with body-local digit_pos
+        loop(pos < end) {
+            local digit_pos = pos - start  // Body-local calculation
+            sum = sum * 10
+            sum = sum + digit_pos          // Use body-local in update
+            pos = pos + 1
+
+            if (sum > 50) {
+                break  // Break condition
+            }
+        }
+
+        print(sum)  // Expected: 0*10+0 → 0*10+1 → 1*10+2 → 12*10+3 → 123 → break
+                    // Output: 123 (breaks before digit_pos=4)
+        return 0
+    }
+}

apps/tests/phase186_p2_body_local_digit_pos_min.hako

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+// Phase 186: Minimal Body-local Init Lowering Test (Pattern2)
+//
+// Tests that body-local variables with init expressions can be used in update expressions.
+//
+// Expected behavior:
+// - loop(pos < 10) iterates with break condition
+// - Each iteration:
+//   - local digit_pos = pos - start (body-local init expression)
+//   - if digit_pos >= 3 { break }
+//   - sum = sum + digit_pos (use body-local in update)
+//   - pos = pos + 1
+// - Expected sum: 0 + (0-0) + (1-0) + (2-0) = 0+0+1+2 = 3
+
+static box Main {
+    main() {
+        local sum = 0
+        local pos = 0
+        local start = 0
+
+        loop (pos < 10) {
+            local digit_pos = pos - start  // Body-local init expression (Phase 186)
+
+            if digit_pos >= 3 {
+                break  // Break when digit_pos reaches 3
+            }
+
+            sum = sum + digit_pos  // Use body-local in update (Phase 184)
+            pos = pos + 1
+        }
+
+        print(sum)  // Expected: 3 (0+0+1+2)
+        return sum
+    }
+}

apps/tests/phase186_p2_body_local_init_simple.hako

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+// Phase 186: Simple Body-local Init Lowering Test (Pattern2)
+//
+// Tests body-local init expressions used ONLY in updates, NOT in conditions.
+// This is within Phase 186 scope (int/arithmetic init, condition-free usage).
+//
+// Expected behavior:
+// - loop(pos < 5) iterates 5 times with break condition on pos
+// - Each iteration:
+//   - local offset = pos - 0 (body-local init: always equals pos)
+//   - if pos >= 3 { break } (condition uses pos, NOT offset)
+//   - sum = sum + offset (use body-local in update)
+//   - pos = pos + 1
+// - Expected sum: 0 + (0-0) + (1-0) + (2-0) = 0+0+1+2 = 3
+
+static box Main {
+    main() {
+        local sum = 0
+        local pos = 0
+
+        loop (pos < 5) {
+            local offset = pos - 0  // Body-local init (Phase 186: BinOp)
+
+            if pos >= 3 {  // Condition uses pos (loop var), NOT offset!
+                break
+            }
+
+            sum = sum + offset  // Update uses offset (body-local)
+            pos = pos + 1
+        }
+
+        print(sum)  // Expected: 3 (0+0+1+2)
+        return sum
+    }
+}