fix(manifests): keep service drm, rebase carried vtt cues, ship single-track mpds

DASH no longer drops service-set DRM when the manifest has none, and the WebVTT rebase allows a cue that starts one segment early. --remote also sends single-track manifests matched by track id.

tests/remote/unit/test_track_manifest_fragments.py

@@ -72,10 +72,46 @@ def test_detached_adaptation_set_round_trips() -> None:
     assert client_track.descriptor == Subtitle.Descriptor.DASH
 
 
+def test_single_track_manifest_round_trips_by_id() -> None:
+    """A manifest that holds only its own track ships as a fragment, so the client needs no attribute match."""
+    manifest = etree.fromstring(MPD.encode())
+    period = manifest.find("Period")
+    adaptation_set = period.find("AdaptationSet")
+    track = Subtitle(
+        id_="abc123",
+        url="https://cdn.example/manifest.mpd",
+        codec=Subtitle.Codec.WebVTT,
+        language=Language.get("en"),
+        sdh=True,
+        descriptor=Subtitle.Descriptor.DASH,
+        data={
+            "dash": {
+                "manifest": manifest,
+                "period": period,
+                "adaptation_set": adaptation_set,
+                "representation": adaptation_set.find("Representation"),
+            }
+        },
+    )
+    client_track = Subtitle(
+        id_="abc123",
+        url=track.url,
+        codec=Subtitle.Codec.WebVTT,
+        language=Language.get("en"),
+        sdh=True,
+        descriptor=Subtitle.Descriptor.URL,
+    )
+    resolve_track_manifests(Tracks([client_track]), extract_track_manifests(Tracks([track])))
+
+    assert client_track.descriptor == Subtitle.Descriptor.DASH
+    assert client_track.data["dash"]["representation_id"] == "sub-0"
+
+
 def test_in_tree_adaptation_set_is_not_shipped() -> None:
     manifest = etree.fromstring(MPD.encode())
     period = manifest.find("Period")
     adaptation_set = period.find("AdaptationSet")
+    adaptation_set.append(etree.fromstring(b'<Representation id="sub-1" bandwidth="2000"/>'))
     track = Subtitle(
         id_="abc123",
         url="https://cdn.example/manifest.mpd",