HS: fix test-456 timeout + add sx_kernel_eval/hs_compile_inspect/hs_parse_inspect tools

- hs-run-filtered.js: add collectionExpressions to _NO_STEP_LIMIT_SUITES (fixes state
  corruption for downstream for-loop tests), add repeat-forever tests to _NO_STEP_LIMIT,
  extend slow deadline for collectionExpressions to 60s
- tests/hs-kernel-eval.js: new standalone Node.js eval script — full WASM kernel +
  mock DOM, accepts HS_EVAL_EXPR/MODE/SETUP/FILES env vars, supports eval/compile/parse modes
- tools/mcp_hs_test.py: add sx_kernel_eval, hs_compile_inspect, hs_parse_inspect tools
- hosts/ocaml/bin/mcp_tree.ml: add host_stubs param to sx_harness_eval (OCaml build pending)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/hs-run-filtered.js

@@ -791,10 +791,13 @@ for(let i=startTest;i<Math.min(endTest,testCount);i++){
     "async hypertrace is reasonable",
     "hypertrace from javascript is reasonable",
     "hypertrace is reasonable",
+    "repeat forever works",
+    "repeat forever works w/o keyword",
   ]);
   // Suites where JIT cascade legitimately exceeds the per-test step limit.
   const _NO_STEP_LIMIT_SUITES = new Set([
     "hs-upstream-core/runtimeErrors",
+    "hs-upstream-expressions/collectionExpressions",
   ]);
   // Enable step limit for timeout protection — reset counter first so accumulation
   // across tests doesn't cause signed-32-bit wraparound (~2B extra steps before limit fires).
@@ -808,6 +811,7 @@ for(let i=startTest;i<Math.min(endTest,testCount);i++){
   };
   const _SLOW_DEADLINE_SUITES = {
     "hs-upstream-core/runtimeErrors": 30000,
+    "hs-upstream-expressions/collectionExpressions": 60000,
   };
   _testDeadline = Date.now() + (_SLOW_DEADLINE[name] || _SLOW_DEADLINE_SUITES[suite] || 10000);
   globalThis.__hs_deadline = _testDeadline; // expose to WASM cek_step_loop