supertensor: shape-aware tensor figure toolkit

Extracted from the tensor-formula-viz skill and rebuilt around the idea that
the geometry rules should be enforced by construction rather than restated as
prose an agent has to remember.

- assets/supertensor.sty: faces, stacks, index faces, shared caption lanes,
  meaning box, signature. Macros take a declared axis and a declared role, so
  equal shapes get equal edges, a x a is square, a transpose swaps the face,
  and contracted axes share an edge length -- without any manual alignment.
- scripts/preflight.sh: decide the TikZ/CJK path before drawing.
- scripts/build.sh: compile and fail on silent corruption (missing CJK glyphs,
  overfull boxes, undeclared roles), then export pdf/svg/png/thumb.
- scripts/test.sh: build every figure as a regression test for the package.
- examples/: three golden figures (TP-FFN, causal MHA, MoE top-k gather) plus
  an anti-pattern gallery of figures that compile cleanly and still lie.
- SKILL.md + references/: lean entry point, details loaded on demand.

scripts/test.sh

@@ -0,0 +1,28 @@
+#!/usr/bin/env bash
+# Build every example and the smoke test. Any dirty build fails the run.
+#
+#   ./scripts/test.sh
+#
+# This is a regression test for assets/supertensor.sty: the examples exercise
+# faces, stacks, index faces, every pattern, shared caption lanes, connectors,
+# the meaning box and the signature.
+set -uo pipefail
+ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+fail=0
+
+for f in "$ROOT"/tests/*.tex "$ROOT"/examples/*.tex; do
+  [[ -e "$f" ]] || continue
+  name="$(basename "$f")"
+  if "$ROOT/scripts/build.sh" "$f" >/dev/null 2>&1; then
+    echo "  ok    $name"
+  else
+    echo "  FAIL  $name"
+    fail=$((fail+1))
+  fi
+done
+
+if [[ $fail -gt 0 ]]; then
+  echo "$fail failing figure(s); rerun scripts/build.sh on one to see why" >&2
+  exit 1
+fi
+echo "all figures build clean"