Add host FFI primitives and web/lib DOM+browser libraries

Introduce 8 irreducible host FFI primitives that replace 40+ native DOM
and browser primitives:

  host-global    — access global object (window/document)
  host-get       — read property from host object
  host-set!      — write property on host object
  host-call      — call method on host object
  host-new       — construct host object
  host-callback  — wrap SX function as host callback
  host-typeof    — check host object type
  host-await     — await host promise

All DOM and browser operations are now expressible as SX library
functions built on these 8 primitives:

  web/lib/dom.sx     — createElement, querySelector, appendChild,
                        setAttribute, addEventListener, classList, etc.
  web/lib/browser.sx — localStorage, history, fetch, setTimeout,
                        promises, console, matchMedia, etc.

The existing native implementations remain as fallback — the library
versions shadow them in transpiled code. Incremental migration: callers
don't change, only the implementation moves from out-of-band to in-band.

JS 957+1080, Python 744, OCaml 952 — zero regressions.

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

web/lib/browser.sx

@@ -0,0 +1,171 @@
+;; ==========================================================================
+;; browser.sx — Browser API library functions
+;;
+;; Location, history, storage, cookies, timers, fetch — all expressed
+;; using the host FFI primitives. Library functions, not primitives.
+;; ==========================================================================
+
+
+;; --------------------------------------------------------------------------
+;; Location & navigation
+;; --------------------------------------------------------------------------
+
+(define browser-location-href
+  (fn ()
+    (host-get (host-get (dom-window) "location") "href")))
+
+(define browser-location-pathname
+  (fn ()
+    (host-get (host-get (dom-window) "location") "pathname")))
+
+(define browser-location-origin
+  (fn ()
+    (host-get (host-get (dom-window) "location") "origin")))
+
+(define browser-same-origin?
+  (fn (url)
+    (starts-with? url (browser-location-origin))))
+
+(define browser-push-state
+  (fn (state title url)
+    (host-call (host-get (dom-window) "history") "pushState" state title url)))
+
+(define browser-replace-state
+  (fn (state title url)
+    (host-call (host-get (dom-window) "history") "replaceState" state title url)))
+
+(define browser-reload
+  (fn ()
+    (host-call (host-get (dom-window) "location") "reload")))
+
+(define browser-navigate
+  (fn (url)
+    (host-set! (host-get (dom-window) "location") "href" url)))
+
+
+;; --------------------------------------------------------------------------
+;; Storage
+;; --------------------------------------------------------------------------
+
+(define local-storage-get
+  (fn (key)
+    (host-call (host-get (dom-window) "localStorage") "getItem" key)))
+
+(define local-storage-set
+  (fn (key val)
+    (host-call (host-get (dom-window) "localStorage") "setItem" key val)))
+
+(define local-storage-remove
+  (fn (key)
+    (host-call (host-get (dom-window) "localStorage") "removeItem" key)))
+
+
+;; --------------------------------------------------------------------------
+;; Timers
+;; --------------------------------------------------------------------------
+
+(define set-timeout
+  (fn (fn-val ms)
+    (host-call (dom-window) "setTimeout" (host-callback fn-val) ms)))
+
+(define set-interval
+  (fn (fn-val ms)
+    (host-call (dom-window) "setInterval" (host-callback fn-val) ms)))
+
+(define clear-timeout
+  (fn (id)
+    (host-call (dom-window) "clearTimeout" id)))
+
+(define clear-interval
+  (fn (id)
+    (host-call (dom-window) "clearInterval" id)))
+
+(define request-animation-frame
+  (fn (fn-val)
+    (host-call (dom-window) "requestAnimationFrame" (host-callback fn-val))))
+
+
+;; --------------------------------------------------------------------------
+;; Fetch
+;; --------------------------------------------------------------------------
+
+(define fetch-request
+  (fn (url opts)
+    (host-call (dom-window) "fetch" url opts)))
+
+(define new-abort-controller
+  (fn ()
+    (host-new "AbortController")))
+
+(define controller-signal
+  (fn (controller)
+    (host-get controller "signal")))
+
+(define controller-abort
+  (fn (controller)
+    (host-call controller "abort")))
+
+
+;; --------------------------------------------------------------------------
+;; Promises
+;; --------------------------------------------------------------------------
+
+(define promise-then
+  (fn (p on-resolve on-reject)
+    (let ((cb-resolve (host-callback on-resolve))
+          (cb-reject (if on-reject (host-callback on-reject) nil)))
+      (if cb-reject
+        (host-call (host-call p "then" cb-resolve) "catch" cb-reject)
+        (host-call p "then" cb-resolve)))))
+
+(define promise-resolve
+  (fn (val)
+    (host-call (host-global "Promise") "resolve" val)))
+
+(define promise-delayed
+  (fn (ms val)
+    (host-new "Promise" (host-callback
+      (fn (resolve)
+        (set-timeout (fn () (host-call resolve "call" nil val)) ms))))))
+
+
+;; --------------------------------------------------------------------------
+;; Dialogs & media
+;; --------------------------------------------------------------------------
+
+(define browser-confirm
+  (fn (msg) (host-call (dom-window) "confirm" msg)))
+
+(define browser-prompt
+  (fn (msg default)
+    (host-call (dom-window) "prompt" msg default)))
+
+(define browser-media-matches?
+  (fn (query)
+    (host-get (host-call (dom-window) "matchMedia" query) "matches")))
+
+
+;; --------------------------------------------------------------------------
+;; JSON
+;; --------------------------------------------------------------------------
+
+(define json-parse
+  (fn (s)
+    (host-call (host-global "JSON") "parse" s)))
+
+
+;; --------------------------------------------------------------------------
+;; Console
+;; --------------------------------------------------------------------------
+
+(define log-info
+  (fn (msg)
+    (host-call (host-global "console") "log" (str "[sx] " msg))))
+
+(define log-warn
+  (fn (msg)
+    (host-call (host-global "console") "warn" (str "[sx] " msg))))
+
+(define now-ms
+  (fn ()
+    (host-call (host-global "Date") "now")))