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>

selfhost/backend/emit_head.ludic

@@ -73,6 +73,16 @@ function emit_header() -> void {
   emith("declare i32 @snd_playing(ptr)\n")
   emith("declare void @snd_set_volume(ptr, i32)\n")
   emith("declare void @snd_set_rate(ptr, i32)\n")
+  emith("declare ptr @hs_req_new(ptr, ptr)\n")          # #6 HTTP transport (http.ll)
+  emith("declare void @hs_req_header(ptr, ptr, ptr)\n")
+  emith("declare void @hs_req_body(ptr, ptr, i32)\n")
+  emith("declare void @hs_send(i32, ptr)\n")
+  emith("declare i32 @hs_done(i32)\n")
+  emith("declare i32 @hs_status(i32)\n")
+  emith("declare ptr @hs_body(i32)\n")
+  emith("declare i32 @hs_blen(i32)\n")
+  emith("declare ptr @hs_header(i32, ptr)\n")
+  emith("declare void @hs_free(i32)\n")
   emith("@__stderrp = external global ptr\n")
   emith("@__stdoutp = external global ptr\n")
   emith("@.fmt_int = private unnamed_addr constant [4 x i8] c\"%d\\0A\\00\"\n")