Single-pass aser_slot for HTMX path + kernel eval timing + stable hash

Eliminated double-aser for HTMX requests: build OOB wrapper AST
(~shared:layout/oob-sx :content wrapped_ast) and aser_slot in ONE
pass — same pattern as the full-page path. Halves aser_slot calls.

Added kernel-side timing to stderr:
  [aser-slot] eval=3.6s io_flush=0.0s batched=3 result=22235 chars

Results show batch IO works (io_flush=0.0s for 3 highlight calls)
and the bottleneck is pure CEK evaluation time, not IO.

Performance after single-pass fix:
  Home: 0.7s eval (was 2.2s total)
  Reactive: 3.6s eval (was 6.8s total)
  Language: 1.1s eval (was 18.9s total — double-aser eliminated)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

hosts/ocaml/bin/dune

@@ -1,3 +1,3 @@
 (executables
  (names run_tests debug_set sx_server)
- (libraries sx))
+ (libraries sx unix))