feat(http): Http.* poll-based HTTP/HTTPS client over a native NSURLConnection backend (#6)

Adds the Http.* namespace and its transport, the HTTP client from #6. The JSON
companion the proposal called for already shipped as Json.* (#44).

- runtime/native/http.ll: the macOS transport — NSURLConnection driven through
  the objc runtime C ABI (no ObjC/C source), run on a detached pthread so the
  frame never blocks; TLS is the system's, on by default. A fixed slot pool holds
  each in-flight request; the worker publishes status/body/response behind an
  atomic done flag (release/acquire). Spliced + Foundation linked only when a
  program uses Http.*.
- runtime/native/http.ludic: the Http.* runtime — get/post/request, the
  open/set/body/send builder, poll/status/ok/text/body_len/header/free, plus a
  pure-Ludic response parser (Http.parse + case-insensitive header lookup) that
  is transport-independent and portable.
- compiler: Http.* dispatch, g_uses_http splice, hs_* transport intrinsics +
  declarations, the conditional Foundation link, and a new \r string/char escape
  the protocol needs.
- docs: a full docs/language/http section (16 pages); check-impl/check-docs green.
- test: examples/library/http.ludic self-asserts the parser offline (Darwin-gated
  build via the canonical path, since it links Foundation).

Verified end to end against real endpoints: HTTPS GET (200 + headers + body) and
POST (body + custom header). HTTP is out-of-band and never feeds the
deterministic sim. Reseeded; suites green (81 + 29).

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

docs/language/http/http-open.md

@@ -0,0 +1,25 @@
+---
+id: http-open
+name: Http.open
+category: http
+kind: namespace-method
+tokens: Http.open
+sig: Http.open(method, url) -> int
+tip: Open a request to configure before sending.
+order: 3
+ns: Http
+member: open
+---
+
+Opens a request without sending it, so headers and a body can be added first with <a href="http-set.html"><code>Http.set</code></a> / <a href="http-body.html"><code>Http.body</code></a>, then dispatched with <a href="http-send.html"><code>Http.send</code></a>.
+
+```ludic
+program Demo {
+  entry {
+    let h = Http.open("POST", "https://example.com/api")
+    Http.set(h, "Authorization", "Bearer token")
+    Http.body(h, "{}")
+    Http.send(h)
+  }
+}
+```