NEON-29: Fix idle jitter on flat treads after stop

idle_support_is_stable no longer treats every post-stop loose-tick frame as
unstable; only slide normals showing floor+wall (lip) keep corrective idle.
Deduplicate ridged-contact check via idle_slide_contacts_are_ridged.
Relax STABLE_IDLE_FLOOR_MIN_UP_DOT slightly for Jolt normal flicker.
Update GdUnit cases and NEON-29 plan note.

client/scripts/player.gd

@@ -22,7 +22,8 @@ const FLOOR_SNAP_IDLE: float = 0.11
 ## Below this floor-normal dot up, idle tick uses **moving** `floor_max_angle` (rim).
 const IDLE_RIM_MIN_FLOOR_UP_DOT: float = 0.968
 ## Stable flat idle support: skip corrective idle motion when support is effectively level.
-const STABLE_IDLE_FLOOR_MIN_UP_DOT: float = 0.9998
+## Slightly below 1.0 so Jolt does not flicker across the threshold on thin treads.
+const STABLE_IDLE_FLOOR_MIN_UP_DOT: float = 0.9996
 ## Rare edge contacts can keep corrective nudges alive forever; budget them, then hold x/z.
 const IDLE_MANUAL_CORRECTION_MAX_TICKS: int = 8
 ## Horizontal nudge per tick for rim / straddle settle (**`_maybe_idle_rim_settle_nudge`**).
@@ -359,14 +360,30 @@ func _physics_idle_tick(delta: float) -> void:
 	move_and_slide()
 
 
+static func idle_slide_contacts_are_ridged(slide_normals: Array[Vector3]) -> bool:
+	var upish := 0
+	var wallish := 0
+	for n: Vector3 in slide_normals:
+		var ny: float = n.y
+		if ny > 0.65:
+			upish += 1
+		elif ny < 0.45:
+			wallish += 1
+	return upish >= 1 and wallish >= 1
+
+
 static func idle_support_is_stable(
-	on_floor: bool, floor_normal: Vector3, _slide_normals: Array[Vector3], loose_ticks: int
+	on_floor: bool, floor_normal: Vector3, slide_normals: Array[Vector3], loose_ticks: int
 ) -> bool:
-	## `_slide_normals` is reserved for future extra-contact analysis.
-	if not on_floor or loose_ticks > 0:
+	if not on_floor:
 		return false
 	if floor_normal.dot(Vector3.UP) < STABLE_IDLE_FLOOR_MIN_UP_DOT:
 		return false
+	# Previously any `_floor_angle_loose_ticks > 0` blocked stable idle for the full post-stop
+	# window (~0.8 s), forcing rim nudges + `move_and_slide` on **flat** approach treads — XZ/Y
+	# jitter next to thin steps. Keep corrective settle only when slides show floor + wall (lip).
+	if loose_ticks > 0 and idle_slide_contacts_are_ridged(slide_normals):
+		return false
 	return true
 
 
@@ -471,15 +488,7 @@ func _notification(what: int) -> void:
 
 
 func _idle_ridged_floor_contacts() -> bool:
-	var upish := 0
-	var wallish := 0
-	for i: int in get_slide_collision_count():
-		var ny: float = get_slide_collision(i).get_normal().y
-		if ny > 0.65:
-			upish += 1
-		elif ny < 0.45:
-			wallish += 1
-	return upish >= 1 and wallish >= 1
+	return idle_slide_contacts_are_ridged(_current_slide_normals())
 
 
 func _maybe_idle_rim_settle_nudge() -> bool:

client/test/player_test.gd

@@ -68,12 +68,18 @@ func test_idle_support_is_stable_true_when_flat_floor_has_extra_contacts() -> vo
 	assert_that(stable).is_true()
 
 
-func test_idle_support_is_stable_false_when_loose_ticks_active() -> void:
-	var slide_normals: Array[Vector3] = [Vector3.UP]
+func test_idle_support_is_stable_false_when_loose_ticks_and_ridged_slide_contacts() -> void:
+	var slide_normals: Array[Vector3] = [Vector3.UP, Vector3(1.0, 0.0, 0.0)]
 	var stable: bool = PLAYER_SCRIPT.idle_support_is_stable(true, Vector3.UP, slide_normals, 12)
 	assert_that(stable).is_false()
 
 
+func test_idle_support_is_stable_true_when_loose_ticks_but_flat_open_support() -> void:
+	var slide_normals: Array[Vector3] = [Vector3.UP]
+	var stable: bool = PLAYER_SCRIPT.idle_support_is_stable(true, Vector3.UP, slide_normals, 12)
+	assert_that(stable).is_true()
+
+
 func test_idle_support_is_stable_false_when_not_on_floor() -> void:
 	var slide_normals: Array[Vector3] = [Vector3.UP]
 	var stable: bool = PLAYER_SCRIPT.idle_support_is_stable(false, Vector3.UP, slide_normals, 0)

docs/plans/NEON-29-implementation-plan.md

@@ -120,6 +120,7 @@ No new automated GDScript tests are added for this story — there is no new GDS
 ## Approach treads (district convention)
 
 - **Shipped in scene:** **`TerracePlatformC_Approach`** (`PCS_*`, teal-style **3×4** cardinals, **7.2 m** NS / **WE** span), **`TerraceStepB_Approach`** (`TSB_*`, **S/E/W** only — north meets violet), **`TerracePlatformB_Approach`** (`TPB_*`, **N/E/W**, **six** ~**0.104 m** risers for **0.6 m** deck height; south remains **gold step**). **Smooth ramps** for other props can follow the same rise/run idea without new `player.gd` logic.
+- **Idle jitter on thin treads:** **`idle_support_is_stable`** used to return false for the entire **`FLOOR_ANGLE_LOOSE_TICKS_AFTER_STOP`** window whenever **`loose_ticks > 0`**, so the capsule ran **rim / corrective** idle on **flat** tread tops after every stop — **`idle_manual_correction`** nudges vs anchor caused visible **XZ/Y** noise. **Fix:** treat as stable during loose ticks unless **`idle_slide_contacts_are_ridged`** (floor + wall in slide normals). Slightly relaxed **`STABLE_IDLE_FLOOR_MIN_UP_DOT`** (**0.9998 → 0.9996**) for Jolt normal flicker.
 - **Not in this pass:** **`PrototypeTerminal`**, **MoveReject** props, **runtime random bumps** — add geometry when those need climbable access.
 
 ## Open questions / risks