docs(pose): fix stale swap note, pin mesh rig

hand_slots module docstring still claimed inverted chirality (default
1) after the live A/B validated chirality correct (default 0); align
it. Pin the MESH_RIG ne0 bool convention (chantier 5 review minor).

data_only_viz/hand_slots.py

@@ -8,10 +8,10 @@ Fallback (chirality absent/misaligned): sort valid hands by screen cx ascending
 (flipped when mirror=True) and assign leftmost to slot 0, next to slot 1.
 
 `swap=True`: inverts the chirality INTERPRETATION only (chir 0 -> right slot,
-1 -> left slot). ARBodyTracker's Vision chirality arrives inverted vs the
-user's mirrored view (observed live 2026-07-02), so HAND_SWAP_LR defaults to
-1 in the consumers; set 0 if a future app build fixes chirality at the
-source. The cx fallback is screen-relative and is never swapped.
+1 -> left slot). Safety knob for a source delivering flipped chirality;
+ARBodyTracker chirality was validated CORRECT live 2026-07-02, so
+HAND_SWAP_LR defaults to 0 in the consumers. The cx fallback is
+screen-relative and is never swapped.
 
 `near_min > 0`: after routing, replace a slot with None when the hand's
 wrist-to-middle-MCP distance (normalised) is below `near_min`.

data_only_viz/tests/test_config.py

@@ -351,3 +351,13 @@ def test_iphone_usb_skips_ensure_model():
     assert ensure_calls == [], (
         f"_ensure_model was called under iphone_usb=True: {ensure_calls}"
     )
+
+
+@pytest.mark.parametrize("val,expected", [
+    ("0", False),
+    ("1", True),
+    ("", True),   # ne0: "" != "0" = True (drift to _bool_std would break this)
+])
+def test_bool_ne0_mesh_rig(val, expected):
+    cfg = VizConfig.from_env({"MESH_RIG": val})
+    assert cfg.mesh_rig is expected