Port from snaporca: the solver's 1024-unknown cliff, and the scale rungs

Two commits carried across (snaporca 579a9a9162, f68613cfc5).

Past about 480 entities a sketch had NO constraints at all and said nothing: libslvs
declares MAX_UNKNOWNS = 1024 and is handed every entity in the sketch at two params
per point, so the whole system came back TOO_MANY_UNKNOWNS and try_add_constraints
rolled the entire inferred batch back. From there no dimension could ever be applied.
Constraints only couple entities that share a point, so the solver now falls back —
only on TOO_MANY_UNKNOWNS — to solving connected components separately and committing
all-or-nothing. The auto-constraint pass batches its Horizontal/Vertical constraints
instead of one solve each, which is what kept the bulk path fast once solves started
succeeding: a 1204-entity load went 1585 ms -> 562 ms.

Plus the scale rungs (a thousand-entity plate drawn on by hand; the heaviest real
drawings graded and timed), the --step 1 fix that used to select nothing while
reporting a clean run, and scripts/ladder-all.sh as the one-command gate.

Parity 17 identical / 8 diverging as expected. Kernel suite here: 188 cases /
2532 assertions, including "a sketch past the solver's unknown limit still solves".

snaporca-yww4, snaporca-x6v7, snaporca-j6sr

scripts/ladder-all.sh

@@ -0,0 +1,52 @@
+#!/usr/bin/env bash
+# Every ladder, in one command, as the gate before a push that touched the Design tab.
+#
+# WHY A SCRIPT AND NOT CI. Three of the four rungs need a running application with an OpenGL
+# canvas and synthetic input; GitHub's runners have neither. So the gate is local and explicit:
+# run this, read the last line, and do not push a red one. The kernel suite is the only part CI
+# can carry, and it already does.
+#
+#   scripts/ladder-all.sh                 # kernel + engine + corpus (every 20th) + gestures
+#   FULL=1 scripts/ladder-all.sh          # corpus over ALL 997 sheets (~25 min)
+#   SKIP_GUI=1 scripts/ladder-all.sh      # kernel only, for a machine with no rig
+#
+# The rig container is expected to be up with the app running and SNAPORCA_MCP set; bring it up
+# with scripts/gui-session.sh inside it. The corpus lives at /corpus in that container.
+set -uo pipefail
+cd "$(dirname "$0")/.."
+
+C="${C:-snaporca-gui}"
+CORPUS="${CORPUS:-/corpus}"
+STEP="${STEP:-20}"
+[ -n "${FULL:-}" ] && STEP=1
+fail=0
+
+step() {
+    local name="$1"; shift
+    echo
+    echo "=== $name ==="
+    if "$@"; then echo "--- $name OK"; else echo "--- $name FAILED"; fail=1; fi
+}
+
+run_in_rig() {                      # copy the script in fresh, then run it there
+    docker cp "$1" "$C:/tmp/$(basename "$1")" >/dev/null || return 1
+    shift
+    docker exec "$C" python3 "$@"
+}
+
+step "kernel suite" scripts/kernel-test.sh --vol "${KVOL:-snaporca_kerneltest}"
+
+if [ -z "${SKIP_GUI:-}" ]; then
+    step "engine ladder (rungs 1-8, scripted geometry)" \
+        run_in_rig scripts/sketch-ladder.py /tmp/sketch-ladder.py
+    step "corpus rung (real drawings, every ${STEP}th)" \
+        run_in_rig scripts/ladder-corpus.py /tmp/ladder-corpus.py --corpus "$CORPUS" --step "$STEP"
+    step "corpus scale rung (the heaviest sheets)" \
+        run_in_rig scripts/ladder-corpus.py /tmp/ladder-corpus.py --corpus "$CORPUS" --scale
+    step "gesture ladder (mouse and keyboard)" \
+        run_in_rig scripts/gui-ladder.py /tmp/gui-ladder.py
+fi
+
+echo
+if [ "$fail" -eq 0 ]; then echo "ALL LADDERS HELD"; else echo "AT LEAST ONE LADDER FAILED"; fi
+exit "$fail"