NEON-29: Stable idle latch + Y anchor for approach treads

Hold latched zero-vel idle through STABLE_IDLE_UNLATCH_TICKS consecutive
unstable readings so thin tread lips do not drop into corrective move_and_slide
every other frame. Idle anchor now freezes Y as well as XZ. Reset latch on
goal/snap/arrival; seed latch on arrival when support is stable after idle tick.
Update hold_idle_anchor GdUnit expectation.

client/scripts/player.gd

@@ -32,6 +32,8 @@ const IDLE_RIM_MIN_FLOOR_UP_DOT: float = 0.968
 const STABLE_IDLE_FLOOR_MIN_UP_DOT: float = 0.999
 ## Rare edge contacts can keep corrective nudges alive forever; budget them, then hold x/z.
 const IDLE_MANUAL_CORRECTION_MAX_TICKS: int = 8
+## Consecutive unstable-idle physics ticks before leaving latched stable idle (thin tread lip).
+const STABLE_IDLE_UNLATCH_TICKS: int = 2
 ## Horizontal nudge per tick for rim / straddle settle (**`_maybe_idle_rim_settle_nudge`**).
 const IDLE_RIM_SETTLE_STEP: float = 0.004
 ## Used by tests and vertical routing checks (feet vs goal surface).
@@ -77,6 +79,10 @@ var _step_assist_cooldown: int = 0
 var _step_assist_active: bool = false
 var _idle_anchor_active: bool = false
 var _idle_anchor_xz: Vector2 = Vector2.ZERO
+var _idle_anchor_y: float = 0.0
+## Latched stable idle: stay on zero-vel + anchor through brief Jolt false negatives on treads.
+var _idle_stable_latched: bool = false
+var _idle_stable_unlatch_streak: int = 0
 var _idle_manual_correction_ticks: int = 0
 var _debug_last_idle_xz: Vector2 = Vector2.INF
 var _debug_idle_heartbeat: int = 0
@@ -103,6 +109,8 @@ func set_authoritative_nav_goal(world_pos: Vector3) -> void:
 	_step_assist_active = false
 	_idle_anchor_active = false
 	_idle_manual_correction_ticks = 0
+	_idle_stable_latched = false
+	_idle_stable_unlatch_streak = 0
 	_walk_nav_column_steering = false
 	_nav_agent.set_target_position(world_pos)
 
@@ -114,6 +122,8 @@ func clear_nav_goal() -> void:
 	_step_assist_active = false
 	_idle_anchor_active = false
 	_idle_manual_correction_ticks = 0
+	_idle_stable_latched = false
+	_idle_stable_unlatch_streak = 0
 	_walk_nav_column_steering = false
 	_nav_agent.set_target_position(global_position)
 
@@ -151,6 +161,8 @@ func snap_to_server(world_pos: Vector3) -> void:
 	_step_assist_active = false
 	_idle_anchor_active = false
 	_idle_manual_correction_ticks = 0
+	_idle_stable_latched = false
+	_idle_stable_unlatch_streak = 0
 	_walk_nav_column_steering = false
 	_nav_agent.set_target_position(settled)
 	reset_physics_interpolation()
@@ -416,9 +428,10 @@ func _hold_idle_anchor() -> void:
 	var pos := global_position
 	if not _idle_anchor_active:
 		_idle_anchor_xz = Vector2(pos.x, pos.z)
+		_idle_anchor_y = pos.y
 		_idle_anchor_active = true
 		return
-	global_position = Vector3(_idle_anchor_xz.x, pos.y, _idle_anchor_xz.y)
+	global_position = Vector3(_idle_anchor_xz.x, _idle_anchor_y, _idle_anchor_xz.y)
 
 
 func _debug_trace_idle_state(tag: String) -> void:
@@ -624,7 +637,17 @@ func _physics_process(delta: float) -> void:
 	)
 	if not _has_walk_goal:
 		floor_block_on_wall = true
-		if _stable_idle_support():
+		var raw_stable: bool = _stable_idle_support()
+		if raw_stable:
+			_idle_stable_unlatch_streak = 0
+			_idle_stable_latched = true
+		elif _idle_stable_latched:
+			_idle_stable_unlatch_streak += 1
+			if _idle_stable_unlatch_streak >= STABLE_IDLE_UNLATCH_TICKS:
+				_idle_stable_latched = false
+				_idle_stable_unlatch_streak = 0
+				_idle_anchor_active = false
+		if _idle_stable_latched:
 			_idle_manual_correction_ticks = 0
 			velocity = Vector3.ZERO
 			floor_snap_length = FLOOR_SNAP_IDLE
@@ -685,10 +708,14 @@ func _physics_process(delta: float) -> void:
 		_step_assist_active = false
 		_idle_anchor_active = false
 		_idle_manual_correction_ticks = 0
+		_idle_stable_latched = false
+		_idle_stable_unlatch_streak = 0
 		_walk_nav_column_steering = false
 		_nav_agent.set_target_position(global_position)
 		_physics_idle_tick(delta)
 		if _stable_idle_support():
+			_idle_stable_latched = true
+			_idle_stable_unlatch_streak = 0
 			_idle_manual_correction_ticks = 0
 			velocity = Vector3.ZERO
 			floor_snap_length = FLOOR_SNAP_IDLE

client/test/player_test.gd

@@ -103,7 +103,7 @@ func test_hold_idle_anchor_sets_anchor_first_then_clamps_xz() -> void:
 
 	p.global_position = Vector3(1.2, 0.7, 2.3)
 	p.call("_hold_idle_anchor")
-	assert_that(p.global_position).is_equal(Vector3(1.0, 0.7, 2.0))
+	assert_that(p.global_position).is_equal(Vector3(1.0, 0.5, 2.0))
 
 
 func test_snap_to_server_clears_idle_anchor() -> void: