feat(phase21.5): MirBuilder optimization prep + crate EXE infrastructure

Phase 21.5 optimization readiness - C-level performance target:
- MirBuilder: JsonFrag purify toggle (HAKO_MIR_BUILDER_JSONFRAG_PURIFY=1)
- Normalizer: extended f64 canonicalization + dedupe improvements
- loop_opts_adapter: JsonFrag path refinement for crate EXE compatibility

Infrastructure improvements:
- provider_registry: add diagnostics + ring-1 providers (array/console/map/path)
- mir_interpreter: add normalization/purify feature gates
- tools/selfhost_exe_stageb.sh: new end-to-end Stage-B→crate EXE pipeline
- tools/perf/microbench.sh: performance measurement tooling

Smoke tests (phase2100):
- Extend timeout 15s→120s for heavy crate EXE builds
- Add stageb_loop_jsonfrag_crate_exe_canary_vm.sh (target test)
- Add s3_backend_selector_crate_exe_vm_parity_return42_canary_vm.sh

Documentation:
- ENV_VARS.md: add Phase 21.5 optimization toggles
- README updates: clarify crate backend strategy
- phase215-optimization.md: new optimization roadmap

This commit sets the stage for Phase 21.5 critical optimization:
achieving C-level performance to decide hakorune's future viability.

tools/smokes/v2/lib/test_runner.sh

@@ -856,3 +856,18 @@ enable_mirbuilder_dev_env() {
     #     export HAKO_MIR_BUILDER_NORMALIZE_TAG=1   # optional: show tags in logs for diagnostics
     # fi
 }
+
+# Dev profile helpers (EXE/AOT bring-up)
+# Sets environment defaults for LLVM crate backend and EXE link paths.
+# Usage: call enable_exe_dev_env in EXE canaries.
+enable_exe_dev_env() {
+    # Prefer crate backend when available
+    export NYASH_LLVM_BACKEND=${NYASH_LLVM_BACKEND:-crate}
+    # Tool locations (override when cross)
+    export NYASH_NY_LLVM_COMPILER=${NYASH_NY_LLVM_COMPILER:-"$NYASH_ROOT/target/release/ny-llvmc"}
+    # NyRT (kernel) lib search path for linking EXEs
+    export NYASH_EMIT_EXE_NYRT=${NYASH_EMIT_EXE_NYRT:-"$NYASH_ROOT/target/release"}
+    # Optional verification toggles (kept ON by default only for canaries that opt-in)
+    export NYASH_LLVM_VERIFY=${NYASH_LLVM_VERIFY:-0}
+    export NYASH_LLVM_VERIFY_IR=${NYASH_LLVM_VERIFY_IR:-0}
+}