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/compile-modules.js

@@ -48,6 +48,8 @@ const SOURCE_MAP = {
   'boot.sx': 'web/boot.sx',
   'tw-layout.sx': 'web/tw-layout.sx', 'tw-type.sx': 'web/tw-type.sx', 'tw.sx': 'web/tw.sx',
   'text-layout.sx': 'lib/text-layout.sx',
+  // Host app components (content-addressed, client-expanded on boosted nav).
+  'relate-picker.sx': 'lib/host/sx/relate-picker.sx',
 };
 let synced = 0;
 for (const [dist, src] of Object.entries(SOURCE_MAP)) {
@@ -87,6 +89,8 @@ const FILES = [
   'hs-tokenizer.sx', 'hs-parser.sx', 'hs-compiler.sx', 'hs-runtime.sx',
   'hs-worker.sx', 'hs-prolog.sx',
   'hs-integration.sx', 'hs-htmx.sx',
+  // Host app components — standalone defcomps, no inter-module deps.
+  'relate-picker.sx',
   'boot.sx',
 ];