feat(sse_client,cli,tui): implement issue #7 — empty-data skip + MalformedSseData

Bundles initial TDD impl + Volva-code-review F1/F3 amendments.

sse_client.py:
- New MalformedSseData(raw) exception; truncates raw to 200 chars at
  __init__ (mirrors MalformedSseId.raw[:64] precedent).
- _iter_events gains `if sse.data == '': continue` BEFORE
  _parse_sse_id. Empty-data frames are silently skipped per issue #7
  INV-001 (keepalive semantics). Empty-data + bad-id is still a
  keepalive; intentional ordering, don't reorder.
- _iter_events json.loads(sse.data) now wrapped — JSONDecodeError →
  MalformedSseData(raw=sse.data).

cli.py:
- Imports MalformedSseData; _run_turn ERROR_ROUTING gains the case →
  stderr `[malformed_sse_data] raw={exc.raw!r}` + exit 22 (protocol-
  failure bucket, same as MalformedSseId/TurnIdFlip).

tui.py:
- Imports MalformedSseData; _stream_turn_worker ERROR_ROUTING gains
  the case → transcript label; finally block restores state→idle
  per INV-008 (mid-session errors don't exit the app).

Tests (6 new):
- test_sse_client.py: empty_data_skipped (tracer — 4 frames in, 3
  events out), malformed_data_raises, whitespace_data_raises,
  malformed_data_truncation, AND empty_data_skip_preserves_last_seen_sse_id
  (F1 from Volva code-review — drop-after-empty probes internal
  last_sse_id non-advancement via SseConnectionDropped.last_seen_sse_id).
- test_cli.py: malformed_sse_data (tightened to assert exact
  `[malformed_sse_data] raw='not-json'` shape per F3),
  malformed_sse_data_truncation (5000-char payload — verifies
  truncation carries through presenter rendering, F3).
- test_tui.py: malformed_sse_data_returns_to_idle (state→idle per
  INV-008; app does NOT exit).

Smoke validation (2026-05-22): the original crashing prompt
("what about system 1 and system 2 framing?") now completes cleanly
end-to-end. mimir streamed 3193 tokens (50 seconds, 374980-token
context), `[done] turn_id=96 duration_ms=50436`. Empty-data frames
somewhere in the stream silently skipped; no crash.

172/172 tests GREEN; ruff clean; all 5 issue contracts (#1, #3, #4,
#5, #7) drift-check clean.

Persistent-memory updated per the commit-along rule: status reflects
v0+#7 milestone; new dated decisions for #5/#6/#7 filing + #7
implementation; foot-gun entry for unguarded json.loads(sse.data).

tests/test_tui.py

@@ -706,6 +706,29 @@ class TestStreamTurnWorker:
             assert app.state == "idle"
             assert any("[connection_dropped]" in str(w) for w in writes)
 
+    @respx.mock
+    async def test_malformed_sse_data_returns_to_idle(
+        self, monkeypatch: pytest.MonkeyPatch
+    ) -> None:
+        """malformed_sse_data_returns_to_idle [error]: bad-JSON → [malformed_sse_data]; idle."""
+        stream = (
+            _sse_chunk("42:1", {"type": "text", "content": "x"})
+            + b"id: 42:2\ndata: not-json\n\n"
+        )
+        respx.post(
+            "https://w.example/sessions/s-1existing/messages"
+        ).mock(return_value=_sse_resp(stream))
+        writes = _spy_writes(monkeypatch)
+        app = RatatoskrApp(_args_existing())
+        async with app.run_test() as pilot:
+            await pilot.pause()
+            await _submit_and_wait(app, pilot, "x")
+            assert app.state == "idle"
+            assert any("[malformed_sse_data]" in str(w) for w in writes)
+            assert any("not-json" in str(w) for w in writes)
+            # INV-008: mid-session error does NOT exit the app
+            assert app.return_value is None
+
     @respx.mock
     async def test_rendered_event_per_event(
         self, monkeypatch: pytest.MonkeyPatch