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/bin/dune

@@ -0,0 +1,3 @@
+(executable
+ (name sx_native_app)
+ (libraries sx sx_native cairo2 tsdl unix))