host: enable serving-mode JIT (SX_SERVING_JIT=1), host code excluded

The Datalog/relations saturation is CPU-bound and JITs cleanly: host conformance
271/271 under JIT, 5.4x faster (1m43s -> 19s, same binary); live /tags 2.5s ->
0.76s. loops/host now carries the merged sx-vm-extensions kernel (the JIT engine
+ gate), built into the binary the container bind-mounts.

- docker-compose: SX_SERVING_JIT=1 (default-OFF gate; opt-in here).
- serve.sh: when JIT is on, (jit-exclude! "host/*" "dream-*" "dr/*"). The host app
  + Dream framework MISCOMPILE on first call in the http-listen + cek_run_with_io
  path (map/rest emit wrong CALL_PRIM args -> 500; the JIT->CEK fallback marks the
  fn failed but does NOT recover the failed call). They're IO-bound, so CEK is no
  slower — but the miscompile is a real kernel-JIT bug to fix upstream (see
  plans/HANDOFF-jit-miscompile.md), after which this exclude can be dropped.

Verified live: cold pages 200 (no first-hit 500), relate picker lists candidates,
relate round-trip works, /tags fast, datalog still JITs (78 dl-* compiles).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docker-compose.dev-sx-host.yml

@@ -29,6 +29,10 @@ services:
       SX_ADMIN_USER: admin
       SX_ADMIN_PASSWORD: "sx-host-camper-van-2026"
       SX_SESSION_SECRET: "ra-host-sess-7c1f9b3e2a8d4056"
+      # Serving-mode JIT: bytecode-compile hot SX (esp. the Datalog/relations path)
+      # on the epoch serving channel. Validated: host conformance 271/271 under JIT,
+      # 5.4x faster (1m43s -> 19s). Default-OFF gate, opt in here.
+      SX_SERVING_JIT: "1"
       OCAMLRUNPARAM: "b"
     volumes:
       # SX source (hot-reload on container restart)

lib/host/serve.sh

@@ -99,6 +99,17 @@ EPOCH=1
   for M in "${MODULES[@]}"; do
     echo "(epoch $EPOCH)"; echo "(load \"$M\")"; EPOCH=$((EPOCH+1))
   done
+  # Serving-mode JIT: exclude the request-path SX (host app + Dream framework) from
+  # JIT. Those functions miscompile on first call in the http-listen + cek_run_with_io
+  # path (a map/rest arg bug → 500, then self-heals via CEK fallback — but the first
+  # hit fails). They're IO-bound anyway, so CEK is no slower. The JIT win is the
+  # Datalog/relations saturation (dl-*/relations-*), which JITs cleanly and stays on
+  # (e.g. /tags 2.5s -> 0.76s).
+  if [ "${SX_SERVING_JIT:-}" = "1" ]; then
+    echo "(epoch $EPOCH)"
+    echo "(eval \"(jit-exclude! \\\"host/*\\\" \\\"dream-*\\\" \\\"dr/*\\\")\")"
+    EPOCH=$((EPOCH+1))
+  fi
   # Point the blog at the DURABLE file backend (persists under $SX_PERSIST_DIR),
   # then idempotently seed a welcome post (sx_content = SX element markup, the
   # editor's content model). Re-seeding is a no-op if the slug already exists.