exp+fix: commit2 增益重扫（gain2.2 到达 50.8%，n=512）+ 浏览器安全层三点修复（限位夹紧/部分前缀/离散认证）

- gain 1.8/2.2/2.5/2.8 = 50.4/52.0/49.6/47.7%（n=256），2.2 最佳；n=512 复核 50.8%/1.4% 碰撞
- 积分反馈与反馈隔次复用均为负结论（−4.0/−5.1 pp）
- JS：tick/候选投影夹 LIM（原越限后 inBox 永久拒绝→刹停）；safety_partial 移植；segCert 开关（默认严格）
- bench nc10 --seg 0：刹车率 79.5→48.1%、planP50 56.1→33.5ms、ok 0→5/10

webapp/js/ik-solver.js

@@ -731,6 +731,23 @@ function make(cfg) {
     for (let i = 0; i < n; i++) M = mul3(M, rotAxis(D[ORDER[i]], q[i] * D2R));
     return M;
   }
+  /* 几何雅可比（对度）：Jv[i][j] = d p_i/d q_j (mm/deg)，Jw[i][j] = 世界角速度分量/deg。
+     J4/J5 同轴不影响正确性：两者各贡献同一世界轴。 */
+  function jac(q) {
+    const { R: Rs, T: Ts } = cumXf(q);
+    const p = fk(q, 0);
+    const Jv = [[], [], []], Jw = [[], [], []];
+    for (let j = 0; j < 6; j++) {
+      const id = ORDER[j], Rj = Rs[j], tj = Ts[j], pj = P[id];
+      const a = mv3(Rj, D[id]);
+      const lever = [p[0] - (Rj[0] * pj[0] + Rj[1] * pj[1] + Rj[2] * pj[2] + tj[0]),
+                     p[1] - (Rj[3] * pj[0] + Rj[4] * pj[1] + Rj[5] * pj[2] + tj[1]),
+                     p[2] - (Rj[6] * pj[0] + Rj[7] * pj[1] + Rj[8] * pj[2] + tj[2])];
+      const v = cross3(a, lever);
+      for (let i = 0; i < 3; i++) { Jv[i][j] = v[i] * D2R; Jw[i][j] = a[i] * D2R; }
+    }
+    return { Jv: Jv, Jw: Jw };
+  }
   /* M5 第 3 列（工具轴）列向量传播：J4/J5 同轴 ⇒ R4·R5=R(θ4+θ5) 合并为一次旋转，
      只乘单位向量、不构造 3×3、免 T3/mul3/rotErrDeg/acos。与 orient(q,5) 列 2 一致（≤1e-15）。 */
   const SAMEAX = D.J4[0] === D.J5[0] && D.J4[1] === D.J5[1] && D.J4[2] === D.J5[2];
@@ -3233,7 +3250,7 @@ function make(cfg) {
     return { poseHit: pHit, poseTried: pTried, poseErr: pErr, poseMs: pTried ? pMs / pTried : 0 };
   }
 
-  return { fk: fk, fkChain: fkChain, solve: solve, solveTo: solveTo, solveWarm: solveWarm, solveDyn: solveDyn,
+  return { fk: fk, fkChain: fkChain, jac: jac, solve: solve, solveTo: solveTo, solveWarm: solveWarm, solveDyn: solveDyn,
            solveRoute: solveRoute, routeFrom: routeFrom, transitPt: transitPt, planAround: planAround, edgeFree: edgeFree, PST: PST, unwrapTo: unwrapTo, rtWarm: rtWarm, rtReset: () => { RT = null; }, rmPlan: rmPlan, rmSmooth: rmSmooth, modelFingerprint: modelFingerprint, RM: RM, wd: wd, classGap: classGap, classFingerprint: classFingerprint, P2D: P2D,
            mixSolve: mixSolve, closest: closest, closest6: closest6, closestOri: closestOri, orient: orient,
            branch: branch, selfTest: selfTest, poseSelfTest: poseSelfTest,