host: Phase 5.1 — interactive SX-page render from a handler, 181/181

KERNEL: add a render-page primitive (sx_server.ml, persistent mode) that renders
an UNEVALUATED SX expression with the server env via sx_render_to_html.
render-to-html expands defcomp components and collects keyword attrs itself; SX
handlers can't reach the server env, so the prim supplies it. Fixes the attr
mangling — bare render-to-html on an EVALUATED component tree turns (form :id ..)
into <form>idpost-new-form..; rendering the unevaluated expr keeps :id an attr.

HOST: lib/host/page.sx — host/page (expr -> HTML response) + host/page-route
(mount on a GET path). New page suite (8 tests) proves a generic attributed +
nested component renders correctly through a host route; verified ~editor/form
renders right too. This is the component-render step of the generic
interactive-SX-page capability; shell + static assets + hydration (5.2-5.4) next.

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

plans/host-on-sx.md

@@ -216,12 +216,18 @@ that pipeline, don't reinvent or patch per-component.
 serving path.
 
 Sub-steps (each independently gated/verified):
-- [ ] **5.1 Page render from a host handler.** Expose the kernel's
-      component-render + shell so an `http-listen` handler can return a full SX
-      page (correct attributes, not the mangling `render-to-html` path). Likely a
-      small `hosts/` addition: a `render-page`-style entry callable from the
-      handler, or a `host/page` route the server renders via the page pipeline.
-      Gate: `~editor/form` (or any attributed component) renders to correct HTML.
+- [x] **5.1 Page render from a host handler.** DONE. Kernel: a `render-page`
+      primitive (sx_server.ml, persistent mode) renders an UNEVALUATED SX
+      expression with the server env via `sx_render_to_html` — render-to-html
+      expands defcomp components + collects keyword attrs itself; SX handlers
+      can't reach the server env, so the prim supplies it. Host: `lib/host/page.sx`
+      — `host/page` (expr → HTML response) + `host/page-route` (mount on a GET
+      path). Gate MET: `~editor/form` renders correct HTML (`<form method="post"
+      class=.. id="post-new-form">…`), and the `page` suite (8 tests) proves a
+      generic attributed+nested component renders right (no `:class`-as-text
+      mangling). Root cause confirmed: bare render-to-html on an *evaluated* tree
+      mangles attrs; `render-page` renders the *unevaluated* expr so expansion +
+      attr-collection happen in render-to-html.
 - [ ] **5.2 Shell statics in the host env.** Run `http_inject_shell_statics` for
       the host's loaded components so the shell can inline defs/CSS/asset-hashes.
       Gate: a full page shell emits with component defs inlined.