feat(trace): live terminal output under bpython via a shared emit()

Live sniff/sim stream trace frames from a background thread. bpython replaces sys.stdout with a capture object whose write() does a greenlet switch back to the main thread — which raises off-thread; the text buffers but only repaints on the next keypress, so streamed frames 'pile up until you touch a key'. New trace/output.py emit() buffers via the repl's on_write and fires bpython's threadsafe repaint trigger; a plain write everywhere else, with a safe fallback on any internals mismatch.

Route TraceFormatter.print(), SimSession's sim-ended status, and SniffSession's state message through emit(). Flip the trace-line newline convention from leading to trailing so each emitted line self-terminates (format_sniff_line + test updated to match).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/test_sim_trace_fmt.py

@@ -232,10 +232,10 @@ class TestTraceFormatterBasic:
         import re
         return re.sub(r'\033\[[0-9;]*m', '', s)
 
-    def test_starts_with_newline(self):
+    def test_ends_with_newline(self):
         fmt = TraceFormatter(mode="sim", decoder=decode_15693)
         result = fmt.format(0, bytes([0x26, 0x01, 0x00]))
-        assert result.startswith("\n")
+        assert result.endswith("\n")
 
     def test_sim_mode_tag(self):
         fmt = TraceFormatter(mode="sim", decoder=decode_15693)