fix(camera): rest-snap render position — kills the indoor doorway standing-still flicker

Root cause (pinned live, flap-churn.log at the Holtburg cottage doorway): the physics
body is byte-stable at rest (rawPlayer = 1 distinct value), but
PlayerMovementController.ComputeRenderPosition's Lerp(prev, curr, alpha) dithers the
render position by microns — the two physics-tick snapshots lag the settled body
(per-frame resolve edge-settles the resting sphere against the doorframe after the last
tick wrote curr) while the leftover-accumulator alpha varies every frame. The grazing-
doorframe camera-collision sweep (PhysicsCameraCollisionProbe.SweepEye) amplifies that
~1000x into a ~1.3 mm eye jitter (eye 17 distinct, RenderPosition 15 distinct) that trips
the PortalVisibilityBuilder clip -> the standing-still flicker (blue void / grass over the
cellar entrance) the user reported.

Fix: at rest (body velocity below RestVelocityEpsilonSq) render AT the authoritative
byte-stable body position instead of interpolating between two stale tick snapshots, so the
camera's pivot input is byte-stable and the sweep output stops jittering. Mirrors retail (a
resting object renders bit-stable) + the boom convergence snap
(RetailChaseCamera.ApplyConvergenceSnap, d2212cf), one layer earlier. Sub-tick interpolation
is preserved during motion (velocity above epsilon).

This SUPERSEDES the committed bounded-propagation plan: the live pin proved ZERO portal
re-enqueue churn during the flap (maxPop=1 across 13k oscillating frames; 0/63k reciprocals
ever clipped empty), so the flap was never the churn the spec hypothesized. The
ACDREAM_PROBE_PORTAL_CHURN apparatus did its job (refuted the hypothesis before the wrong
fix was built); plan/spec/memory updates to follow.

TDD: extracted the rest-snap into an internal-static pure ComputeRenderPosition; RED rest-
snap test (stale prev!=curr + varying alpha dithers) -> GREEN after the gate; motion test
guards interpolation; precondition test confirms a settled body's velocity is below the
gate threshold. 29 controller+cellar + 62 camera+portal tests green, no regression.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/AcDream.App/Input/PlayerMovementController.cs

@@ -810,7 +810,30 @@ public sealed class PlayerMovementController
     private Vector3 ComputeRenderPosition()
     {
         float alpha = Math.Clamp(_physicsAccum / PhysicsBody.MinQuantum, 0f, 1f);
-        return Vector3.Lerp(_prevPhysicsPos, _currPhysicsPos, alpha);
+        return ComputeRenderPosition(_prevPhysicsPos, _currPhysicsPos, _body.Position, _body.Velocity, alpha);
+    }
+
+    // Render-position rest-snap (2026-06-08, indoor doorway flap). At rest the authoritative
+    // body position is byte-stable, but the two physics-tick snapshots (prev/curr) can lag it by
+    // microns — the per-frame resolve edge-settles the resting sphere against doorframe geometry
+    // after the last tick wrote curr — so Lerp(prev, curr, alpha) with a per-frame-VARYING
+    // leftover-accumulator alpha dithers the render position by microns. The grazing-doorframe
+    // camera-collision sweep (PhysicsCameraCollisionProbe.SweepEye) amplifies that ~1000x into a
+    // ~1.3 mm eye jitter that trips the portal-flood clip → the standing-still indoor flicker
+    // (pinned live, flap-churn.log: rawPlayer 1 distinct, RenderPosition 15 distinct, eye 17).
+    // When the body is at rest (velocity below epsilon) render AT the authoritative position so
+    // the camera's pivot input is byte-stable. Mirrors retail (a resting object renders
+    // bit-stable) + the boom convergence snap (RetailChaseCamera.ApplyConvergenceSnap, d2212cf),
+    // one layer earlier. Interpolation between tick snapshots is preserved during motion
+    // (velocity above epsilon), so sub-tick movement stays smooth.
+    internal const float RestVelocityEpsilonSq = 1e-4f;  // (0.01 m/s)^2 — below this the body is at rest
+
+    internal static Vector3 ComputeRenderPosition(
+        Vector3 prevPhysicsPos, Vector3 currPhysicsPos, Vector3 bodyPosition, Vector3 bodyVelocity, float alpha)
+    {
+        if (bodyVelocity.LengthSquared() < RestVelocityEpsilonSq)
+            return bodyPosition;  // at rest: render at the authoritative byte-stable position
+        return Vector3.Lerp(prevPhysicsPos, currPhysicsPos, alpha);
     }
 
     public MovementResult Update(float dt, MovementInput input)

tests/AcDream.Core.Tests/Input/PlayerMovementControllerTests.cs

@@ -224,6 +224,73 @@ public class PlayerMovementControllerTests
         Assert.Equal(result.Position.Z, result.RenderPosition.Z, precision: 4);
     }
 
+    // ── Indoor doorway flap: render-position rest-snap (2026-06-08) ───────────
+    //
+    // Live pin (flap-churn.log, user at the cottage doorway): the physics body is
+    // byte-stable at rest (rawPlayer = 1 distinct value), but the render position
+    // (Lerp of the two physics-tick snapshots) jitters ~µm and the camera EYE
+    // jitters ~1.3 mm — a ~1000x amplification by the grazing-doorframe camera-
+    // collision sweep, which trips the portal clip → the standing-still flicker.
+    // The dither is structural: at rest the tick snapshots (_prevPhysicsPos /
+    // _currPhysicsPos) can lag the settled authoritative Position by microns (door-
+    // frame edge-settle in the per-frame resolve), so Lerp(prev, curr, alpha) with a
+    // per-frame-VARYING alpha sweeps a tiny segment instead of holding still. Fix:
+    // at rest (velocity below epsilon) render AT the authoritative body position —
+    // byte-identical and alpha-independent. Mirrors retail (a resting object renders
+    // bit-stable) and the boom convergence snap (RetailChaseCamera.ApplyConvergenceSnap,
+    // d2212cf), one layer earlier so the camera's pivot input is byte-stable too.
+    //
+    // The flat-terrain controller tests above CANNOT reproduce the doorframe-specific
+    // prev!=curr-at-rest condition (flat terrain collapses prev==curr), so these test
+    // the pure rest-snap function directly; the end-to-end acceptance is the live
+    // doorway visual gate.
+    [Fact]
+    public void ComputeRenderPosition_AtRestWithStaleEndpoints_SnapsToAuthoritativePosition_NoAlphaDither()
+    {
+        // prev lags curr by 30 µm (the live doorframe edge-settle lag); body = the settled
+        // authoritative position; velocity = 0 (at rest). Two different leftover-accumulator
+        // alphas must BOTH return the authoritative position, byte-identical (no dither).
+        var prev = new Vector3(155.525116f, 14.225600f, 94f);
+        var curr = new Vector3(155.525146f, 14.225600f, 94f);
+        var body = new Vector3(155.525146f, 14.225600f, 94f);
+
+        var lowAlpha  = PlayerMovementController.ComputeRenderPosition(prev, curr, body, Vector3.Zero, alpha: 0.15f);
+        var highAlpha = PlayerMovementController.ComputeRenderPosition(prev, curr, body, Vector3.Zero, alpha: 0.93f);
+
+        Assert.Equal(body, lowAlpha);       // byte-identical to the authoritative position
+        Assert.Equal(lowAlpha, highAlpha);  // alpha-independent at rest (no dither)
+    }
+
+    [Fact]
+    public void ComputeRenderPosition_Moving_InterpolatesBetweenTickSnapshots()
+    {
+        // Guard the no-over-fire half: while moving (velocity well above the rest epsilon)
+        // the render position must still interpolate smoothly between the tick snapshots.
+        var prev = new Vector3(96.0f, 96f, 50f);
+        var curr = new Vector3(96.3f, 96f, 50f);
+        var moving = new Vector3(3.12f, 0f, 0f);  // walk speed
+
+        var half = PlayerMovementController.ComputeRenderPosition(prev, curr, curr, moving, alpha: 0.5f);
+
+        Assert.Equal(96.15f, half.X, precision: 3);  // midpoint — interpolation preserved
+    }
+
+    [Fact]
+    public void Update_AtRest_BodyVelocityBelowRenderRestSnapThreshold()
+    {
+        // Precondition for the render-position rest-snap: a settled grounded body's velocity must
+        // be below RestVelocityEpsilonSq, else ComputeRenderPosition's gate never fires at rest and
+        // the doorway flicker persists. kill_velocity on grounded contact drives it to zero.
+        var engine = MakeFlatEngine();
+        var controller = new PlayerMovementController(engine);
+        controller.SetPosition(new Vector3(96f, 96f, 50f), 0x0001);
+        for (int i = 0; i < 60; i++) controller.Update(1f / 60f, new MovementInput());
+
+        Assert.True(
+            controller.BodyVelocity.LengthSquared() < PlayerMovementController.RestVelocityEpsilonSq,
+            $"resting body velocity {controller.BodyVelocity.Length()} m/s must be below the rest-snap threshold");
+    }
+
     [Fact]
     public void Update_RunForward_MoveFasterThanWalk()
     {