Implement Seriatim transcript parsing

internal/modules/input/seriatim/testdata/duplicate_segment_id.json

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+{
+  "metadata": {
+    "id": "duplicate-segment-fixture"
+  },
+  "segments": [
+    {
+      "id": "seg-001",
+      "start": 0,
+      "end": 1,
+      "speaker": "Narrator",
+      "text": "First segment."
+    },
+    {
+      "id": "seg-001",
+      "start": 1,
+      "end": 2,
+      "speaker": "Player",
+      "text": "Duplicate segment."
+    }
+  ]
+}

internal/modules/input/seriatim/testdata/valid_minimal.json

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+{
+  "metadata": {
+    "id": "session-alpha",
+    "source_id": "fallback-session",
+    "title": "Synthetic session transcript"
+  },
+  "segments": [
+    {
+      "id": "seg-001",
+      "start": 0,
+      "end": 4.5,
+      "speaker": "Narrator",
+      "text": "The stone door opens."
+    },
+    {
+      "id": "seg-002",
+      "start": 4.5,
+      "end": 8,
+      "speaker": "Player",
+      "text": "I cast light."
+    }
+  ]
+}