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.

references/layout.md

@@ -4,11 +4,29 @@ Everything on the canvas is a bounding box: tensors, full offset stacks, bracket
 operators, arrow labels, annotations, symbols, shape labels, stage headings, the meaning
 box, the signature. **Tangency counts as collision.**
 
+## Place by cursor, not by coordinate
+
+The invariants below are *statements about the finished figure*; the flow layout in
+`api.md` is how you get them without checking each one by hand. Use it by default:
+`\ststage` / `\strow` … `\strowend` / `\stcol` … `\stcolend`, with an empty coordinate
+argument on every face.
+
+The failure it removes is specific. With hand-written offsets, each gap is a magic number
+tuned against the *current* content, so the day a label grows by two characters it silently
+lands on the next tensor — the figure still compiles and still looks clean. With the
+cursor, every object reserves its own width, so growing one object can only push the rest
+apart. And because a band declares its height, an object that does not fit becomes a
+`Package supertensor Warning`, which `build.sh` turns into a failed build.
+
+Hand placement remains available for the cases the cursor cannot express. When you use it,
+wrap the node in `\sttrack` so the bounding box and the vertical cursor still see it.
+
 ## Gutters
 
-Define one base gutter `g ≥ 1 em`. Unrelated boxes stay at least `g` apart; stage bands at
-least `1.5g`. In practice: `right=5mm–9mm` between an operator and its operands, `7mm–9mm`
-between the last caption of one row and the next stage heading.
+Define one base gutter `g ≥ 1 em`; that is what `\stgutter` (6 mm) is. Unrelated boxes stay
+at least `g` apart; stage bands at least `1.5g` (`\strowgap`, `\stblockgap`). Set them once
+at the top of the figure rather than per call — a per-call `gap=` is for stating a
+*relationship* (`gap=0pt` = these shards tile), not for nudging.
 
 Overlap is allowed only inside one declared composite:
 
@@ -33,26 +51,22 @@ shapes           <- \stcaption arg 3
 ```
 
 Faces of different heights would otherwise hang their captions at different depths. Fix it
-with a shared baseline:
+with a shared baseline — `\strowend` arms one automatically:
 
 ```tex
-\node[inner sep=0pt, fit=(Q)(KT)(S)] (rowA) {};
-\stlane{rowA}
+\strow{rowA}{T}
+  ...
+\strowend
 \stcaption{Q}{$\mathbf Q^{(i)}$}{$h\times T\times d_h$}
-...
-\stnolane
 ```
 
-Every `\stcaption` between `\stlane` and `\stnolane` hangs from the bottom of `rowA`, so
-symbols and shapes form two flat lanes.
+Under absolute placement, arm it yourself with `\stlane{rowA}` … `\stnolane`. Either way
+every `\stcaption` inside hangs from the bottom of `rowA`, so symbols and shapes form two
+flat lanes.
 
-Stage headings share one left rail. Anchor each heading below the previous row but at the
-previous *heading's* x, not at the previous row's content:
-
-```tex
-\coordinate (cy) at ($(I-shape.south)+(0,-9mm)$);
-\node[st stage, anchor=north west] (SC) at (SB.west |- cy) {...};
-```
+Stage headings share one left rail. `\ststage` puts them there: the rail is a single stored
+`x` (`\stleftrail` to move it), and the `y` is derived from the lowest ink drawn so far, so
+a heading can neither drift right nor collide with the row above it.
 
 Explanatory prose belongs in the stage subtitle, the bottom box, or above its own
 connector. Never drop a floating commentary card between two operands unless it is a real
@@ -67,8 +81,10 @@ never cover a face. Two consequences you still own:
 - A connector may not cross a box that is not one of its endpoints. Move the row, don't
   route over.
 - A label's white underlay may cover only its own connector — never a tensor, never
-  another label. If the label is wider than the arrow, shorten the label or widen the gap.
-  This is the single most common failure after a first draft.
+  another label. This is the single most common failure after a first draft, and `\stlink`
+  is the fix: it makes the label itself a flow object and draws the arrow to whatever
+  lands on either side of it, so the label cannot be wider than its connector.
+  `\starrowlabel` between two hand-placed nodes still has the old failure mode.
 
 ## Stacks