feat(profiler/behave_shell): emit environmental.terminal_multiplexer

Scans RAW output (multiplexer escapes are themselves ANSI; never
strip first) for tmux markers (DCS passthrough, focus-reporting,
window-title with tmux marker) and screen markers (DCS, screen-OSC).
Detected → tmux/screen at 0.85; otherwise → none at 0.55. Skips
emission entirely when no commands — silence on a pure-echo or
empty session, per the smoke gates.

When both detected (nested mux), prefer tmux.

tests/profiler/behave_shell/test_environmental_terminal_multiplexer.py

@@ -0,0 +1,74 @@
+"""Step F.2: ``environmental.terminal_multiplexer``."""
+from __future__ import annotations
+
+from decnet.profiler.behave_shell import extract_session
+from decnet.profiler.behave_shell._parse import AsciinemaEvent
+
+
+PRIMITIVE = "environmental.terminal_multiplexer"
+
+
+def _of(observations: list, primitive: str):
+    obs = [o for o in observations if o.primitive == primitive]
+    assert len(obs) == 1, f"expected exactly one {primitive}, got {len(obs)}"
+    return obs[0]
+
+
+def _typed(text: str, t0: float = 0.0, dt: float = 0.05) -> list[AsciinemaEvent]:
+    return [(t0 + i * dt, "i", c) for i, c in enumerate(text)]
+
+
+def test_clean_pty_emits_none() -> None:
+    events: list[AsciinemaEvent] = [
+        *_typed("ls\r"),
+        (0.20, "o", "file1\nfile2\n"),
+    ]
+    out = list(extract_session(events, sid="mux-clean"))
+    obs = _of(out, PRIMITIVE)
+    assert obs.value == "none"
+
+
+def test_tmux_dcs_passthrough_detected() -> None:
+    events: list[AsciinemaEvent] = [
+        *_typed("ls\r"),
+        (0.20, "o", "\x1bPtmux;passthrough_payload\x1b\\"),
+    ]
+    obs = _of(list(extract_session(events, sid="mux-tmux-dcs")), PRIMITIVE)
+    assert obs.value == "tmux"
+
+
+def test_tmux_focus_reporting_detected() -> None:
+    events: list[AsciinemaEvent] = [
+        *_typed("ls\r"),
+        (0.20, "o", "\x1b[?1004h"),  # set focus reporting
+    ]
+    obs = _of(list(extract_session(events, sid="mux-tmux-focus")), PRIMITIVE)
+    assert obs.value == "tmux"
+
+
+def test_screen_dcs_detected() -> None:
+    events: list[AsciinemaEvent] = [
+        *_typed("ls\r"),
+        (0.20, "o", "\x1bP=value\x1b\\"),
+    ]
+    obs = _of(list(extract_session(events, sid="mux-screen")), PRIMITIVE)
+    assert obs.value == "screen"
+
+
+def test_both_present_prefers_tmux() -> None:
+    """Nested mux setup — prefer tmux (more common)."""
+    events: list[AsciinemaEvent] = [
+        *_typed("ls\r"),
+        (0.20, "o", "\x1bPtmux;\x1b\\\x1bP=screen\x1b\\"),
+    ]
+    obs = _of(list(extract_session(events, sid="mux-both")), PRIMITIVE)
+    assert obs.value == "tmux"
+
+
+def test_none_has_lower_confidence_than_detected() -> None:
+    """``none`` could be a hidden multiplexer; confidence reflects that."""
+    none_events: list[AsciinemaEvent] = _typed("ls\r") + [(0.20, "o", "file1\n")]
+    tmux_events: list[AsciinemaEvent] = _typed("ls\r") + [(0.20, "o", "\x1bPtmux;\x1b\\")]
+    n = _of(list(extract_session(none_events, sid="mux-n")), PRIMITIVE)
+    t = _of(list(extract_session(tmux_events, sid="mux-t")), PRIMITIVE)
+    assert n.confidence < t.confidence