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-solidification/README.md

@@ -36,7 +36,13 @@ Canaries
 - tools/dev/stageb_loop_json_canary.sh — Program(JSON) shape for loop(i<n){i=i+1}
 - tools/dev/phase216_chain_canary.sh — end‑to‑end EXE rc=10 for minimal loop
 
+Provider Path Notes (Dev)
+- Optional normalization for provider output is available via `HAKO_MIR_NORMALIZE_PROVIDER=1`.
+  - This applies the same JsonFrag normalizer/purifier to MIR(JSON) emitted by the Rust Provider path.
+  - Keep defaults unchanged; use only during bring‑up to eliminate ret‑after effects.
+- `HAKO_MIR_BUILDER_LOOP_FORCE_JSONFRAG=1` now short‑circuits both selfhost‑first and provider‑first wrappers to emit a minimal, pure control‑flow MIR suitable for EXE build sanity.
+  - Default OFF; intended for small canaries and performance harness bring‑up.
+
 Removal Plan for temporary parser fallback
 - Once VM/gpos interaction is fixed and parser emits correct loop JSON without guards,
   remove the conservative fallback in ParserControlBox.parse_loop.
-