Add native SX desktop browser — renders s-expressions to pixels

A 5.9MB OCaml binary that renders SX pages directly using SDL2 + Cairo,
bypassing HTML/CSS/JS entirely. Can fetch live pages from sx.rose-ash.com
or render local .sx files.

Architecture (1,387 lines of new code):
  sx_native_types.ml   — render nodes, styles, layout boxes, color palette
  sx_native_style.ml   — ~40 Tailwind classes → native style records
  sx_native_layout.ml  — pure OCaml flexbox (measure + position)
  sx_native_render.ml  — SX value tree → native render nodes
  sx_native_paint.ml   — render nodes → Cairo draw commands
  sx_native_fetch.ml   — HTTP fetch via curl with SX-Request headers
  sx_native_app.ml     — SDL2 window, event loop, navigation, scrolling

Usage:
  dune build                           # from hosts/native/
  ./sx_native_app.exe /sx/             # browse sx.rose-ash.com home
  ./sx_native_app.exe /sx/(applications.(native-browser))
  ./sx_native_app.exe demo/counter.sx  # render local file

Features:
  - Flexbox layout (row/column, gap, padding, alignment, grow)
  - Tailwind color palette (stone, violet, white)
  - Rounded corners, borders, shadows
  - Text rendering with font size/weight
  - Click navigation (links trigger refetch)
  - Scroll with mouse wheel
  - Window resize → re-layout
  - URL bar showing current path

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

hosts/native/lib/sx_native_fetch.ml

@@ -0,0 +1,37 @@
+(** HTTP fetcher for SX pages.
+
+    Uses curl via Unix.open_process_in for simplicity.
+    Fetches pages from sx.rose-ash.com with SX-Request headers. *)
+
+let base_url = "https://sx.rose-ash.com"
+
+(** Fetch a URL and return the response body as a string. *)
+let fetch_url (url : string) : string =
+  let cmd = Printf.sprintf
+    "curl -s -L -H 'Accept: text/sx' -H 'SX-Request: true' '%s'" url in
+  let ic = Unix.open_process_in cmd in
+  let buf = Buffer.create 8192 in
+  (try while true do Buffer.add_char buf (input_char ic) done
+   with End_of_file -> ());
+  ignore (Unix.close_process_in ic);
+  Buffer.contents buf
+
+(** Fetch an SX page by path (e.g. "/sx/" or "/sx/language"). *)
+let fetch_page (path : string) : string =
+  let url = if String.length path > 0 && path.[0] = '/' then
+    base_url ^ path
+  else if String.length path > 4 && String.sub path 0 4 = "http" then
+    path
+  else
+    base_url ^ "/" ^ path
+  in
+  fetch_url url
+
+(** Read a local .sx file. *)
+let read_file (path : string) : string =
+  let ic = open_in path in
+  let n = in_channel_length ic in
+  let buf = Bytes.create n in
+  really_input ic buf 0 n;
+  close_in ic;
+  Bytes.to_string buf