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>

src/mir/builder.rs

@@ -442,6 +442,12 @@ impl MirBuilder {
             };
 
             if let Some(block) = function.get_block_mut(block_id) {
+                // Invariant: Call must always carry a Callee (unified path).
+                if let MirInstruction::Call { callee, .. } = &instruction {
+                    if callee.is_none() {
+                        return Err("builder invariant violated: MirInstruction::Call.callee must be Some (unified call)".into());
+                    }
+                }
                 if utils::builder_debug_enabled() {
                     eprintln!(
                         "[BUILDER] emit @bb{} -> {}",