Add server-initiated-external-source scenario (#61)

README.md

@@ -15,6 +15,7 @@ Current scenarios:
 - `server-initiated-burst-cadence` (Server observes client/time burst cadence): start the server first, then the client. The server counts `client/time` messages over a fixed observation window and asserts the distribution matches the recommended cadence (bursts of 8, ~10 s apart). No SDK in the matrix declares the `client-time-burst-cadence` capability yet, so every case currently fails fast and the matrix surfaces the gap
 - `server-initiated-request-format` (Client requests a mid-stream codec switch): start the server first, then the client. The server begins streaming in Opus; the client is instructed to emit `stream/request-format` with FLAC at a fixed timestamp and the server responds with a fresh `stream/start`. No SDK currently emits `stream/request-format`, so every case currently fails fast on the `stream-request-format-emit` capability
 - `server-initiated-pcm-24bit` (Server initiates connection and client wants 24-bit PCM): start the server first, then the client. The server re-packs the fixture as 24-bit PCM (3-byte packed, little-endian, two's complement). No SDK in the matrix declares the `pcm-24bit-decode` capability yet, so every case currently fails fast and the matrix surfaces the gap (notably the `sendspin-go` SDK has no 24-bit code path per the audit)
+- `server-initiated-external-source` (Client enters and leaves `external_source` mid-stream): start the server first, then the client. The client transitions to `external_source` and back; the harness verifies the server emits the right `group/update` and `stream/end` and that the previous group is restored. Per the audit only SendspinKit exposes a client-side API; no client in the matrix declares the `external-source-client-api` capability yet, so every case fails fast
 
 ## Current coverage
 

src/conformance/implementations.py

@@ -66,6 +66,7 @@
                 "client-time-burst-cadence",
                 "stream-request-format-emit",
                 "pcm-24bit-decode",
+                "external-source-client-api",
             ),
         ),
     ),

src/conformance/scenarios.py

@@ -196,6 +196,25 @@
 )
 
 
+SERVER_INITIATED_EXTERNAL_SOURCE = ScenarioSpec(
+    id="server-initiated-external-source",
+    display_name="Client enters and leaves `external_source` mid-stream",
+    description=(
+        "Start the server first, then the client. The client enters "
+        "`external_source` mid-stream; the harness asserts the server emits the right "
+        "`group/update` and `stream/end`; then the client leaves `external_source` and "
+        "the previous group is restored. Per the audit, only SendspinKit exposes a "
+        "client-side API today, so every other client fails fast on the "
+        "`external-source-client-api` capability."
+    ),
+    initiator_role="server",
+    preferred_codec="pcm",
+    required_role_families=("player",),
+    verification_mode="capability-only",
+    required_capability="external-source-client-api",
+)
+
+
 SERVER_INITIATED_REQUEST_FORMAT = ScenarioSpec(
     id="server-initiated-request-format",
     display_name="Client requests a mid-stream codec switch",
@@ -227,6 +246,7 @@
     SERVER_INITIATED_BURST_CADENCE,
     SERVER_INITIATED_REQUEST_FORMAT,
     SERVER_INITIATED_PCM_24BIT,
+    SERVER_INITIATED_EXTERNAL_SOURCE,
 )
 
 SCENARIOS: dict[str, ScenarioSpec] = {scenario.id: scenario for scenario in SCENARIO_LIST}