Body step 2.1: climbing, mantling, crawling, legs, playground, 144 Hz

- Step rule + lower-body collider: no walking onto tables, steps <= 0.35 m
- Structural hand strength: two-hand mantle/pull-up works, one hand can't
- Arms aim from the physical shoulder horizontally, keep True Body pose vertically
- Crawl pose, crawl carry, 35 deg slope limit, ground feed-forward ignores
  play-space corrections (fixes a runaway feedback loop)
- Grab static world (climbing), procedural visual legs
- Playground sandbox; menu strength slider, spectator (old drag) and turn toggles
- Physics 144 Hz with interpolation; legacy chase-body spikes pin 90 Hz
- RagdollSpike: 27 phases incl. mantle, pull-up, crawl, ledge drop, ramps

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

scripts/TrueBody.cs

@@ -14,6 +14,10 @@ public sealed class TrueBodySolver
     public const float ChestLeanMax = 0.75f;              // rad (~43°) at a deep crouch
     public const float CrouchRange = 0.5f;                // crouch factor 1 when eyes are this fraction of standing eye height lower
     public const float HeadPitchToChest = 0.25f;
+    // Crawl: when the head gets so low that the hips would go through the floor, the trunk tips
+    // further (pivoting at the hips, hips kicking back) up to nearly horizontal (Nimso: lower = crawl).
+    public const float CrawlLeanMax = 1.45f;              // rad (~83°)
+    public const float HipMinHeight = 0.15f;              // hip joint above the floor when crawling
     public const float PelvisFrequency = 2.5f;            // Hz, critically damped follow → hip lag/sway
     public const float GirdleReachStart = 0.98f;          // fraction of arm length where the shoulder starts to help
     public const float GirdleSmoothRate = 15f;
@@ -100,18 +104,27 @@ public sealed class TrueBodySolver
         Crouch = Mathf.Clamp((standingPivot - pivotAboveFloor) / (CrouchRange * p.EyeHeight), 0f, 1f);
         float headPitchDown = Mathf.Max(0f, -Mathf.Asin(Mathf.Clamp(headFwd.Y, -1f, 1f)));
         float lean = Crouch * ChestLeanMax + headPitchDown * HeadPitchToChest;
-        Lean = lean;
-        ChestBasis = yawBasis * new Basis(Vector3.Right, -lean);
-
-        // Neck: from the head pivot down along the average of chest-up and head-up, sized so a
-        // standing, level head puts the shoulder joints at the measured drop below the eyes.
+        float trunk = TrunkRatio * p.Stature;
         float neckLen = Mathf.Max(0.05f, p.ShoulderDrop + EyeToNeckPivot.Y - NeckBaseAboveShoulders);
-        Vector3 neckDir = (ChestBasis.Y + headBasis.Y).Normalized();
-        NeckBase = NeckPivot - neckDir * neckLen;
-        // Keep the shoulder line behind the eyes by the anthropometric offset (neck pivot already
-        // supplies part of it).
-        UpperChest = NeckBase - ChestBasis.Y * NeckBaseAboveShoulders
-            + ChestBasis * new Vector3(0f, 0f, ShouldersBehindEyes - EyeToNeckPivot.Z);
+        Vector3 neckDir = Vector3.Up;
+        for (int iter = 0; iter < 3; iter++)
+        {
+            ChestBasis = yawBasis * new Basis(Vector3.Right, -lean);
+            // Neck: from the head pivot down along the average of chest-up and head-up, sized so a
+            // standing, level head puts the shoulder joints at the measured drop below the eyes.
+            neckDir = (ChestBasis.Y + headBasis.Y).Normalized();
+            NeckBase = NeckPivot - neckDir * neckLen;
+            // Keep the shoulder line behind the eyes by the anthropometric offset (neck pivot already
+            // supplies part of it).
+            UpperChest = NeckBase - ChestBasis.Y * NeckBaseAboveShoulders
+                + ChestBasis * new Vector3(0f, 0f, ShouldersBehindEyes - EyeToNeckPivot.Z);
+            float room = (UpperChest.Y - floorY - HipMinHeight) / trunk;
+            float crawl = Mathf.Acos(Mathf.Clamp(room, Mathf.Cos(CrawlLeanMax), 1f));
+            if (crawl <= lean + 1e-3f)
+                break;
+            lean = crawl;
+        }
+        Lean = lean;
 
         // Pelvis: hangs below the upper chest along the trunk; follows through a critically damped
         // spring so it lags quick head motion (hip sway) instead of being rigidly attached.