Phase 21.7 normalization: optimization pre-work + bench harness expansion

- Add opt-in optimizations (defaults OFF)
  - Ret purity verifier: NYASH_VERIFY_RET_PURITY=1
  - strlen FAST enhancement for const handles
  - FAST_INT gate for same-BB SSA optimization
  - length cache for string literals in llvmlite
- Expand bench harness (tools/perf/microbench.sh)
  - Add branch/call/stringchain/arraymap/chip8/kilo cases
  - Auto-calculate ratio vs C reference
  - Document in benchmarks/README.md
- Compiler health improvements
  - Unify PHI insertion to insert_phi_at_head()
  - Add NYASH_LLVM_SKIP_BUILD=1 for build reuse
- Runtime & safety enhancements
  - Clarify Rust/Hako ownership boundaries
  - Strengthen receiver localization (LocalSSA/pin/after-PHIs)
  - Stop excessive PluginInvoke→BoxCall rewrites
- Update CURRENT_TASK.md, docs, and canaries

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

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

apps/examples/json_query/main.hako

@@ -67,7 +67,7 @@ static box Main {
   }
 
   // Evaluate a simple JSON path by slicing JSON text directly (no full parse)
-  // Returns a JSON substring for the value or null if not found
+  // Returns a JSON substring for the value or the string "null" if not found
   eval_path_text(json_text, path) {
     local DEBUG = 0  // set to 1 for ad-hoc debug
     local cur_text = json_text
@@ -87,11 +87,11 @@ static box Main {
         }
         local key = path.substring(start, i)
         if DEBUG == 1 { print("[dbg] key=" + key) }
-        if key.length() == 0 { return null }
+        if key.length() == 0 { return "null" }
         // Get value text directly; then reset window to that text
         local next_text = this.object_get_text(cur_text, 0, cur_text.length(), key)
         if DEBUG == 1 { if next_text == null { print("[dbg] obj miss") } else { print("[dbg] obj hit len=" + next_text.length()) } }
-        if next_text == null { return null }
+        if next_text == null { return "null" }
         cur_text = next_text
       } else {
         if ch == "[" {
@@ -102,16 +102,16 @@ static box Main {
           loop(i < path.length() && this.is_digit(path.substring(i, i + 1))) { i = i + 1 }
           local idx_str = path.substring(start, i)
           if DEBUG == 1 { print("[dbg] idx_str=" + idx_str + ", next=" + path.substring(i, i + 1)) }
-          if i >= path.length() || path.substring(i, i + 1) != "]" { return null }
+          if i >= path.length() || path.substring(i, i + 1) != "]" { return "null" }
           i = i + 1 // skip ']'
           local idx = this.parse_int(idx_str)
           if DEBUG == 1 { print("[dbg] idx=" + idx) }
           local next_text = this.array_get_text(cur_text, 0, cur_text.length(), idx)
           if DEBUG == 1 { if next_text == null { print("[dbg] arr miss idx=" + idx_str) } else { print("[dbg] arr hit len=" + next_text.length()) } }
-          if next_text == null { return null }
+          if next_text == null { return "null" }
           cur_text = next_text
         } else {
-          return null
+          return "null"
         }
       }
     }