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>

lib/host/static.sx

@@ -93,9 +93,16 @@
 
 ;; the data-sx-manifest JSON for the shell: {"modules": {"dom.sxbc": "hash", ...}}.
 ;; The client's loadBytecodeFile reads manifest.modules[file] -> hash -> /sx/h/.
+;; App components the client must eager-load (after the web stack) so their
+;; defcomps are registered before a boosted fragment references them. Loaded
+;; content-addressed via the modules map below, the same as any web-stack module.
+(define host/static--boot-modules (list "relate-picker.sxbc"))
+
 (define host/static-manifest-json
   (fn ()
-    (str "{\"v\":1,\"boot\":[],\"defs\":{},\"modules\":{"
+    (str "{\"v\":1,\"boot\":["
+      (join "," (map (fn (m) (str "\"" m "\"")) host/static--boot-modules))
+      "],\"defs\":{},\"modules\":{"
       (join ","
         (map (fn (k) (str "\"" k "\":\"" (get host/static--file->hash k) "\""))
           (keys host/static--file->hash)))