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.

references/checklist.md

@@ -0,0 +1,59 @@
+# Pre-delivery checklist
+
+A clean `build.sh` proves only that TeX was happy. Nothing below is checked by the compiler.
+Work through it against the rendered PNG. Any mandatory violation means redraw, not patch.
+
+## 1. Math (before looking at the picture)
+
+- [ ] Every shape recomputed independently from the source formula or code.
+- [ ] Block multiplication, broadcasting, reductions and sharding algebra verified.
+- [ ] Axis identities that the figure asserts actually hold in the numbers drawn
+      (`d = h·d_h`, `Σ_e n_e = T·k`, shards summing to the parent).
+
+## 2. Semantics ledger
+
+- [ ] Every discrete or overloaded symbol has one type, domain and range.
+- [ ] Score, index and mask are three distinct objects in three distinct grammars.
+- [ ] Producer-to-consumer chain closed: scores → indices → gather/mask → values.
+- [ ] Shared selectors drawn once, with the reuse axis marked.
+
+## 3. Geometry ledger
+
+- [ ] Equal shapes have identical faces everywhere.
+- [ ] `a×a` is square; every transpose physically swaps the face.
+- [ ] Both occurrences of each contracted axis have the same edge length.
+- [ ] Shards tile their parent exactly; concat reverses split; elisions use an ellipsis.
+
+## 4. Full-size visual audit
+
+Open the PNG at 100 %.
+
+- [ ] No forbidden intersection, tangency, clipping or occlusion — including stack offset
+      sheets, brackets, arrow labels and the meaning box.
+- [ ] Every connector's white label underlay covers only its own connector.
+- [ ] No connector crosses a box that is not its endpoint.
+- [ ] Symbols and shapes sit on two flat lanes per row; stage headings share a left rail.
+- [ ] Top zone compact (≤2 formula lines, no shape underbraces).
+- [ ] Bottom box: one column, ≤3 rows, no overflow, no shrunken type.
+- [ ] Signature outside the box, one line, names what the figure actually shows, not clipped
+      and not visually dominant.
+- [ ] Structural support exact: known zeros unfilled, masks and diagonals exactly right.
+
+## 5. Thumbnail audit
+
+Open `*-thumb.png` (360 px).
+
+- [ ] ≤4 active hue families per row plus gray; each role keeps one hue across stages.
+- [ ] Base colors still muted, not saturated.
+- [ ] Three visibly separated lightness levels where values vary; nothing is uniform pastel.
+- [ ] Borders neutral, not tensor-colored.
+- [ ] Dense texture reads as noise, not as stripes, checkerboard or a symmetric motif.
+- [ ] The main structural claim of the figure is still legible at this size.
+
+## 6. Delivery
+
+- [ ] PNG preview shown.
+- [ ] Short mechanism explanation in prose.
+- [ ] `.tex` source and vector artifact (PDF/SVG) linked.
+- [ ] Any degraded path stated explicitly — no CJK font, no `pdftocairo`, fallback renderer,
+      inferred shapes or conventions.