fix(subscribestar): initial feed GET is a navigation, not XHR (first-run drift)

First live run (cheunart) tripped the drift guard: "no posts and no recognizable
feed container". The browser-saved page was normal (6 posts + posts_container-list),
so the parser was fine — our live HTTP fetch got a different response. Cause: the
client set X-Requested-With: XMLHttpRequest (+ a JSON Accept) session-wide, so the
initial creator-page GET was sent as an XHR. SubscribeStar (Rails) content-
negotiates an XHR full-page request to a non-HTML body → no container → drift.

Fix: the session now uses browser-like navigation headers (Accept: html, NO
X-Requested-With); the XHR header + JSON Accept are applied PER-REQUEST only on
the "load more" endpoint (which is a genuine XHR). Drift message now reports the
response length + a JSON hint so a recurrence is self-explaining. Regression test
pins the header split (navigation initial GET, XHR load-more).

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

tests/test_subscribestar_native.py

@@ -77,6 +77,33 @@ def test_parse_ss_datetime_variants():
     assert _parse_ss_datetime("nonsense") is None
 
 
+# -- client: request headers (regression: live-fetch drift) -----------------
+
+def test_initial_fetch_is_navigation_loadmore_is_xhr(monkeypatch):
+    # The initial creator-page GET must NOT be flagged XHR (Rails content-
+    # negotiates an XHR full-page request to a non-HTML body → drift; first live
+    # run, cheunart 2026-06-17). Only the "load more" endpoint is a real XHR.
+    client = SubscribeStarClient(None)
+    seen = []
+
+    class _R:
+        status_code = 200
+        text = '<div data-role="posts_container-list"></div>'
+
+        def json(self):
+            return {"html": ""}
+
+    def fake_get(url, *, headers=None):
+        seen.append(headers)
+        return _R()
+
+    monkeypatch.setattr(client, "_get", fake_get)
+    client._feed_html("https://subscribestar.adult/x")
+    client._loadmore_html("https://subscribestar.adult/posts?page=2")
+    assert seen[0] is None  # navigation — no per-request XHR header
+    assert seen[1]["X-Requested-With"] == "XMLHttpRequest"
+
+
 # -- client: iteration + pagination -----------------------------------------
 
 def test_iter_posts_parses_and_paginates(monkeypatch):