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.

src/backend/mir_interpreter/exec.rs

@@ -257,6 +257,11 @@ impl MirInterpreter {
             if Self::trace_enabled() {
                 eprintln!("[vm-trace] inst bb={:?} {:?}", block.id, inst);
             }
+            // Dev counters: count non-phi instructions and compares
+            self.inst_count = self.inst_count.saturating_add(1);
+            if let MirInstruction::Compare { .. } = inst {
+                self.compare_count = self.compare_count.saturating_add(1);
+            }
             self.execute_instruction(inst)?;
         }
         Ok(())
@@ -281,6 +286,8 @@ impl MirInterpreter {
                 then_bb,
                 else_bb,
             }) => {
+                // Dev counter: count branch terminators actually evaluated
+                self.branch_count = self.branch_count.saturating_add(1);
                 let cond = self.reg_load(*condition)?;
                 let branch = to_bool_vm(&cond).map_err(VMError::TypeError)?;
                 let target = if branch { *then_bb } else { *else_bb };