Add flow layout: cursor placement, left rail, declared band heights

Figures were positioned by hand-written offsets. Every gap was a magic
number tuned against the content that happened to be there, so a label
that grew two characters landed on the next tensor, and two stages
started from two different x shared no rail. Both failures compile
cleanly.

Replace it with a cursor. Objects placed with an empty coordinate
argument reserve their own width -- including a stack's offset sheets
and a bracket's overhang -- and gaps are declared once (\stgutter,
\strowgap, \stblockgap). The gap belongs to the object that follows it
and the first object in a band gets none, so every band starts flush on
a shared rail and gap=0pt states that two shards tile exactly.

\stlink makes a connector's label a flow object, which is what removes
the label-wider-than-its-arrow failure entirely. \stcol is a vertical
sub-flow for a split along the contracted axis. \strow declares its
height, so an object that does not fit -- or a column that does not add
up to what it declared, and is therefore drawn off-center -- becomes a
package warning, which build.sh fails on.

Absolute placement is unchanged: passing a coordinate takes the original
code path, and \sttrack folds a hand-placed node back into the cursor.

All three golden examples and the new tests/flow.tex are converted and
build clean.

README.md

@@ -37,14 +37,14 @@ A minimal figure:
 \stdim{d}{4}
 
 \begin{document}\begin{tikzpicture}
-  \stface[role=act, bracket=true]{X}{(0,0)}{T}{d}
-  \node[st op, right=6mm of X] (m) {$\times$};
-  \stface[role=w]{W}{($(m)+(1.4,0)$)}{d}{d}
-  \node[inner sep=0pt, fit=(X)(W)] (row) {};
-  \stlane{row}
+  \ststage{S1}{one band, placed by cursor}
+  \strow{row}{T}                            % band height, declared once
+    \stface[role=act, bracket=true]{X}{}{T}{d}   % empty coord = at the cursor
+    \stglyph{m}{$\times$}
+    \stface[role=w]{W}{}{d}{d}
+  \strowend
   \stcaption{X}{$\mathbf X$}{$T\times d$}
   \stcaption{W}{$\mathbf W$}{$d\times d$}
-  \stnolane
 \end{tikzpicture}\end{document}
 ```
 
@@ -52,13 +52,19 @@ Because `T` and `d` come from the ledger, the contracted axis is automatically o
 length in both operands, `d×d` is automatically square, and any other face of shape `T×d`
 in the figure is automatically identical to `X`.
 
+Because the coordinates are empty, each object reserves its own width and the gap between
+them is `\stgutter`, declared once. Nothing here is a tuned offset, so growing a label can
+only push its neighbours apart — it can never land on top of one. And the band declares its
+height, so an object that does not fit is a failed build rather than something the reader
+discovers.
+
 See `references/api.md` for the full macro list.
 
 ## Examples
 
 | file | shows |
 |---|---|
-| `tp-ffn-allreduce.tex` | column-then-row sharding, exact tiling, one hue per TP rank, a collective as a real node |
+| `tp-ffn-allreduce.tex` | column-then-row sharding, exact tiling, one hue per TP rank, a collective as a real node, a `\stcol` split along the contracted axis |
 | `mha-causal.tex` | leading axes as stack depth, a physically swapped `Kᵀ`, a mask in a different grammar from the scores it gates |
 | `moe-topk-gather.tex` | scores → indices → Boolean support → gather, with all three cell grammars side by side |
 | `antipatterns.tex` | four figures that compile cleanly and still teach something false |
@@ -77,6 +83,10 @@ Two LaTeX warnings produce a figure that is quietly wrong rather than visibly br
 greps for both and exits non-zero. A `Package supertensor Warning` — an undeclared role
 falling back to gray — is treated the same way.
 
+The flow layout adds two of its own: an object that overflows its band, and a `\stcol`
+whose contents do not add up to the height it declared (which means it is drawn
+off-center). Both are things a reader would have to notice for you.
+
 A clean build still proves nothing about collisions, hue budget or whether the math is
 right. That is what `references/checklist.md` is for.