sx-http: static file serving + inline CSS + cache-bust hashes

OCaml HTTP server now serves /static/* files with MIME types, immutable
cache headers, and in-memory caching. Computes MD5 hashes for JS/WASM
files and injects them as ?v= cache-busting query params.

Inline CSS: reads tw.css + basics.css at startup, injects into
<style id="sx-css"> tag. Pages now render with full Tailwind styling.

Shell statics now include real file hashes:
  sx-js-hash, body-js-hash, wasm-hash — 12-char MD5 hex

Docker compose: mounts shared/static as /app/static for the container.

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

docker-compose.dev-sx-native.yml

@@ -12,3 +12,6 @@ services:
       SX_SPEC_DIR: /app/spec
       SX_LIB_DIR: /app/lib
       SX_WEB_DIR: /app/web
+    volumes:
+      # Static files (CSS, JS, WASM) — served by Caddy on externalnet
+      - ./shared/static:/app/static:ro