fix(content): put the Saignee entrance landmark where the groove is deep

The landmark sat on GROOVE_START, where the groove's end taper brings
its depth to zero, so the marker (-34 m) was below the seabed (-26.5 m)
and a probe there saw no trench. It now sits two half-widths inside the
groove, on the groove floor.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

content/sites/zone1_vigilant.tres

@@ -91,7 +91,7 @@ position_m = Vector3(0, -25, -230)
 [sub_resource type="Resource" id="Resource_u8dqp"]
 script = ExtResource("1_n3xpu")
 placement_id = &"saignee_entrance"
-position_m = Vector3(-60, -34, -140)
+position_m = Vector3(-56.978443, -35.474903, -124.287895)
 
 [sub_resource type="Resource" id="Resource_3uttw"]
 script = ExtResource("1_n3xpu")