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/antipatterns.md

@@ -15,9 +15,9 @@ dimension. **Fix:** swap the arguments — `\ststack{KT}{...}{dh}{T}{3}`. See `g
 ## 2. Shards that do not tile their parent
 
 Two shards drawn with a gap, or stretched to fill a parent whose other shards were elided.
-Both assert a width that the tensor does not have. **Fix:** place each shard from the
-previous one's edge (`($(W1a.east)+(2*\stunit,0)$)`), and draw an ellipsis for anything
-omitted. See `geometry.md` §5–6.
+Both assert a width that the tensor does not have. **Fix:** draw the shards in one band
+and give every shard after the first `gap=0pt`, which *states* that they are adjacent
+instead of arranging for it; draw an ellipsis for anything omitted. See `geometry.md` §5–6.
 
 ## 3. An index drawn as a heatmap
 
@@ -41,12 +41,18 @@ See `style.md`.
   reads the resulting stripe as real structure. `pattern=dense` avoids it; if you write
   your own filler, check that rows 1, 2, 4, 5 of a tall face are not identical.
 - **A label wider than its connector.** The white underlay then covers the target tensor.
-  Shorten the label or widen the gap — never let it sit on a face. See `layout.md`.
+  `\stlink` makes this unrepresentable: the label reserves its own width in the band and
+  the arrow is drawn to whatever lands beside it. See `layout.md`.
 - **A new hue for a regrouped view of the same data.** `X` and the per-expert buffers
   gathered out of `X` are the same object in a different order; a second hue claims they
   are different tensors.
 - **Captions hanging at different depths** because the faces in a row have different
-  heights. Use `\stlane`.
+  heights. Use `\strow`/`\strowend`, which arms `\stlane` for you.
+- **Hand-tuned offsets.** Each one is a magic number valid only for the content that was
+  there when you tuned it; the figure that breaks is the *next* one, when a label grows two
+  characters and lands on a face. Use the cursor. See `layout.md`.
+- **A formula line placed first.** It can only be centered on a figure whose width is not
+  known yet, so it ends up visibly off-center. Call `\sttopformula` after the bands.
 - **A floating commentary card between two operands.** If it is not a real operation, it
   belongs in the stage subtitle or the bottom box.
 - **A meaning box that repeats the shapes.** The shapes are already under every block. The