Fix uninitialized read in bezier9MaxCurvature when called via bezier9InitFast

bezier9MaxCurvature iterated the dκ/ds and 9D dκ/ds loops up to
BEZIER9_ARC_LENGTH_SAMPLES (1024) regardless of how many samples were
actually populated.  bezier9InitFast fills only SAMPLES_FAST (32) entries
of t_table[]/s_table[], leaving slots [33..1024] holding uninitialized
stack memory from the caller.

Reading those slots produced garbage max_dkappa_ds values that depended
on prior call-stack contents.  Since max_dkappa_ds feeds the centripetal
jerk cap v_jerk = cbrt(j_max * dt / max_dkappa_ds) inside the blend
optimizer, the resulting blend v_plan was history-dependent: the same
gcode produced different execution times depending on what tolerance the
previous program used (different prior runs left different stack patterns,
creating multiple stable timing fixed points).

Fix: clamp the loop bound to b->arc_samples for both the 3D and 9D
curvature-rate scans, with a safety fallback to the full size if the
field is out of range.

Introduced by 43516ac "Fast bezier init for blend optimizer".

Author: Luca Toniolo

src/emc/motion_planning/bezier9.cc

@@ -768,9 +768,17 @@ int bezier9MaxCurvature(Bezier9 * const b)
     // Compute max |dκ/ds| using arc-length table nodes.
     // The curvature rate determines the centripetal jerk = v³ · dκ/ds,
     // which must be bounded for smooth motion at blend entry/exit.
+    //
+    // Loop bound MUST be b->arc_samples, not BEZIER9_ARC_LENGTH_SAMPLES.
+    // bezier9InitFast populates only SAMPLES_FAST entries; reading past that
+    // returns uninitialized stack memory and produces non-deterministic
+    // max_dkappa_ds values that depend on prior call-stack contents.
+    int n_samples = b->arc_samples;
+    if (n_samples <= 0 || n_samples > BEZIER9_ARC_LENGTH_SAMPLES)
+        n_samples = BEZIER9_ARC_LENGTH_SAMPLES;
     double max_dkds = 0.0;
     double prev_kappa = bezier9Curvature(b, 0.0);
-    for (int i = 1; i <= BEZIER9_ARC_LENGTH_SAMPLES; i++) {
+    for (int i = 1; i <= n_samples; i++) {
         double kappa_i = bezier9Curvature(b, b->t_table[i]);
         double ds = b->s_table[i] - b->s_table[i - 1];
         if (ds > BEZIER9_MIN_LENGTH) {
@@ -788,9 +796,10 @@ int bezier9MaxCurvature(Bezier9 * const b)
     // axes change rapidly through the blend, max_dkappa_ds (xyz-only) is
     // near zero while the actual joint-space jerk from the Jacobian-
     // amplified rotary curvature is large.  max_dkappa_ds_9d captures this.
+    // Same loop-bound rule as above.
     double max_dkds_9d = 0.0;
     double prev_kappa_9d = bezier9Curvature9D(b, 0.0);
-    for (int i = 1; i <= BEZIER9_ARC_LENGTH_SAMPLES; i++) {
+    for (int i = 1; i <= n_samples; i++) {
         double kappa_9d_i = bezier9Curvature9D(b, b->t_table[i]);
         double ds_i = b->s_table[i] - b->s_table[i - 1];
         if (ds_i > BEZIER9_MIN_LENGTH) {