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>

docs/development/roadmap/phases/phase-21.6/README.md

@@ -0,0 +1,51 @@
+# Phase 21.6 — Dual‑Emit Parity & C‑line Readiness
+
+Goal: Produce identical MIR(JSON) from both provider (Rust) and selfhost (Hako) builder paths, measure generation cost, and keep AOT (ny‑llvmc) fast/green. All work is semantics‑preserving; defaults remain unchanged.
+
+## Checklists
+
+- [ ] Dual‑emit parity on representative apps (MIR(JSON) normalized SHA1 equal)
+- [ ] Resolver‑first ON passes quick/integration
+- [ ] Selfhost‑first fallback ok (provider/legacy on failure)
+- [ ] AOT obj/exe via ny‑llvmc (crate backend) green
+- [ ] Docs updated (bench guides, env vars, quick recipes)
+
+## Scripts
+
+- Dual emit + compare + bench: `tools/perf/dual_emit_compare.sh <input.hako> [rounds]`
+- MIR emit bench: `tools/perf/bench_hakorune_emit_mir.sh <input.hako> [rounds]`
+- AOT bench: `tools/perf/bench_ny_mir_builder.sh <mir.json> [rounds]`
+- MIR diff: `tools/perf/compare_mir_json.sh <a.json> <b.json>`
+
+## Env Knobs
+
+- `HAKO_USING_RESOLVER_FIRST=1` (resolver‑first)
+- `HAKO_SELFHOST_BUILDER_FIRST=1` (selfhost→provider→legacy)
+- `HAKO_MIR_BUILDER_BOX=hako.mir.builder|min`
+- `NYASH_LLVM_BACKEND=crate`（ny‑llvmc）
+- `HAKO_LLVM_OPT_LEVEL=0|1`（AOT O0 既定）
+
+## Benchmarks — Tracking
+
+Record normalized parity and generation times here (edit in place).
+
+Legend: SHA1 = normalized JSON digest; Parity=Yes when SHA1 equal; Times are medians unless noted.
+
+| Benchmark (Hako)                         | Resolver | Parity | Provider p50 (ms) | Selfhost p50 (ms) | Notes |
+|------------------------------------------|----------|--------|-------------------|-------------------|-------|
+| apps/examples/json_query/main.hako       | off/on   |        |                   |                   |       |
+| apps/examples/json_pp/main.hako          | off/on   |        |                   |                   |       |
+| apps/examples/json_lint/main.hako        | off/on   |        |                   |                   |       |
+| apps/examples/json_query_min/main.hako   | off/on   |        |                   |                   |       |
+
+How to fill:
+1) Run `tools/perf/dual_emit_compare.sh <file> 5`
+2) Copy p50s from the summary lines and mark Parity based on `compare_mir_json.sh` output.
+3) Note any diffs (callee kinds/order/phi/meta) in Notes.
+
+## Next Steps
+
+- [ ] If parity holds on the above set, extend to apps/tests subset
+- [ ] If diffs remain, categorize and align either provider or selfhost output
+- [ ] Keep AOT line green under `HAKO_LLVM_OPT_LEVEL=0` and optional `=1` spot checks
+