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/checklist.md

@@ -26,8 +26,13 @@ Work through it against the rendered PNG. Any mandatory violation means redraw,
 
 ## 4. Full-size visual audit
 
-Open the PNG at 100 %.
+Open the PNG at 100 %. Most of the first three items are automatic under the flow layout;
+they still need looking at, because the cursor only guarantees that objects do not *push*
+into each other, not that the figure reads correctly.
 
+- [ ] No absolute coordinate that a `\strow`/`\stcol` could have expressed. Every remaining
+      hand-placed node is wrapped in `\sttrack`.
+- [ ] `\sttopformula` called after the bands, so the formula is centered on the real figure.
 - [ ] 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.