host: extract the relate picker into a content-addressed ~relate-picker component

The declarative picker markup is now a reusable SX component
(lib/host/sx/relate-picker.sx, defcomp ~relate-picker &key slug kind) instead of
inline markup in the editor. It is a CONTENT-ADDRESSED, CLIENT-EXPANDED component:

- Server: on a full page load render-page expands ~relate-picker server-side
  (SEO / no-JS), exactly as before.
- Client: on a boosted SPA nav the edit body serialises to the compact
  (~relate-picker :slug … :kind …), and the CLIENT expands it. The component
  module is compiled to a content-addressed .sxbc, served immutably from
  /sx/h/{hash}, and listed in the page's data-sx-manifest "boot" array so the
  client eager-loads it after the web stack — registering its defcomp before any
  boosted fragment references it.

Wiring:
- lib/host/sx/relate-picker.sx — the component.
- lib/host/blog.sx — editor emits (~relate-picker :slug s :kind k); the inline
  form markup is gone.
- lib/host/static.sx — host/static-manifest-json emits boot:["relate-picker.sxbc"]
  (the previously-empty boot array, now used as designed).
- hosts/ocaml/browser/sx-platform.js — loadWebStack eager-loads the page manifest's
  boot[] modules (content-addressed) after the web stack.
- bundle.sh + compile-modules.js — copy/compile the component to .sxbc.
- serve.sh + conformance.sh — load the component module server-side.

This gives the host an app-component system: app defcomps shipped to the client by
hash, the same machinery as the kernel modules — the picker is the first, and it's
the model for publishing components externally.

Tests: conformance 272/272 (server expansion); relate-picker.spec.js 6/6 incl. the
boosted-nav populate (proves client-side component load + expansion) and the
error/retry case. WASM stack rebuilt (relate-picker.sxbc @ 6818110a).

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

hosts/ocaml/browser/bundle.sh

@@ -71,6 +71,11 @@ cp "$ROOT/shared/sx/templates/tw-layout.sx" "$DIST/sx/"
 cp "$ROOT/shared/sx/templates/tw-type.sx"   "$DIST/sx/"
 cp "$ROOT/shared/sx/templates/tw.sx"        "$DIST/sx/"
 
+# 9b. Host app components (content-addressed, client-expanded on boosted nav).
+# Listed in the host's data-sx-manifest "boot" array so the client eager-loads
+# them after the web stack — see lib/host/static.sx + sx-platform.js loadWebStack.
+cp "$ROOT/lib/host/sx/relate-picker.sx" "$DIST/sx/"
+
 # 10. Hyperscript
 for f in tokenizer parser compiler runtime integration htmx; do
   cp "$ROOT/lib/hyperscript/$f.sx" "$DIST/sx/hs-$f.sx"