host: per-request IO kernel fix + fully-dynamic blog (no cache), 159/159

KERNEL (sx_server.ml): route http-listen handlers through cek_run_with_io
instead of bare Sx_runtime.sx_call, so handlers resolve per-request IO
(durable persist reads/writes) via the same IO-driving runner the REPL uses.
Verified: per-request read+write, 10 concurrent writes (15 on disk, no
corruption), handler errors don't crash the server, http contract 6/6.

BLOG: fully dynamic — host/blog-post reads the post from the durable store
(content/head) AND renders (content/html) per request, no in-memory view, no
cached output. Possible because of the IO fix. Honest ~2s due to interpreted
Smalltalk render.

Render speed is NOT solved here: the JIT (precompiler) isn't installed in the
serving mode and currently miscompiles the Smalltalk evaluator's nested ASTs
(enabling it breaks ~60% of tests). Fixing the JIT is a separate, high-payoff
effort. Documented in the plan.

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

lib/host/serve.sh

@@ -104,12 +104,8 @@ EPOCH=1
   echo "(eval \"(host/blog-use-store! (persist/durable-backend))\")"
   EPOCH=$((EPOCH+1))
   echo "(epoch $EPOCH)"
-  # Materialise any persisted posts into the in-memory view, then ensure the
-  # welcome post exists (idempotent). Both run on the main thread (IO is fine
-  # here; request handlers only read the view).
-  echo "(eval \"(host/blog-load-all!)\")"
-  EPOCH=$((EPOCH+1))
-  echo "(epoch $EPOCH)"
+  # Idempotently seed the welcome post into the durable store (no-op if present).
+  # Handlers read + render from the store per request (per-request IO).
   echo "(eval \"(host/blog-seed! \\\"welcome\\\" \\\"Welcome to the SX host\\\" \\\"This page is rendered by lib/host on the SX runtime, persisted in the SX store — no Quart.\\\" 1)\")"
   EPOCH=$((EPOCH+1))
   echo "(epoch $EPOCH)"