poster cards: measure the density the Driver asked about

The Driver asked for "around 4 per row on desktop, like Instagram". The grid is
ONE rule for every width -- repeat(auto-fill, minmax(210px,1fr)) -- so the count
falls out of the viewport rather than being chosen, and nothing measured it. The
card's own verify says so: "perRow is currently printed nowhere and asserted
nowhere. A number printed but not asserted on is decoration."

MEASURED, all five widths:
    390px   max 1/row @ 354px
    768px   max 3/row @ 224px
   1024px   max 4/row @ 225px   <- already on target
   1440px   max 5/row @ 212px
   1920px   max 5/row @ 212px
The miss is only at 1440 and 1920, and the card width explains it: the grid
packs a fifth card by SHRINKING to 212px rather than growing toward 225px,
because auto-fill adds a column the moment 5x210 fits.

⚠ REPORTED, NOT ASSERTED, deliberately. The desktop target is settled (~4); the
MOBILE one is not -- 3 per row at 390px is a ~110px card, less than half the
224px that currently holds a readable print footer at 768px. Asserting a number
the Driver has not ruled on would encode my taste as a requirement. Measuring it
is what makes the ruling answerable, and it is the precondition for tuning:
you cannot tune toward a target you are not measuring.

★ IT IS THE MAX ACROSS ROWS, NOT THE FIRST ROW. Grouping every card by top edge
at 1440px gives [5,5,5,3,3,2,2,1,1] -- the short rows are the REMAINDERS of
collapsible sections, not a density failure. Reading the first row alone is
right here by luck and wrong on a wall whose first section holds one poster.

MUTATION-TESTED so the number cannot be decoration itself: widening the track to
minmax(260px,1fr) moves it to 1/2/3/4/4 per row at 354/351/309/273/273px. That
also prices wp1 -- 260px lands 1440 and 1920 on exactly 4 with BIGGER cards
(273px vs 212px), which is what "a bit bigger, more room for the footer" asked
for, at the cost of dropping 768px to 2/row. Which is why that card says "add a
desktop breakpoint" rather than "change the number". Not shipped: it changes
what readers see, and the card is gated.

internal/check-poster-card-fits.mjs

@@ -222,10 +222,40 @@ async function measure(width, lang) {
     // what its buttons SAY has not been fixed. scrollWidth > clientWidth is the browser telling us
     // the text did not fit its box.
     const clipped = ctrls.filter((e) => e.scrollWidth > e.clientWidth + 1)
+    // ★ DENSITY: how many posters share a row, and how wide each is.
+    // The Driver asked for "around 4 per row on desktop, like Instagram". The grid is ONE rule for
+    // every width — repeat(auto-fill, minmax(210px,1fr)) — so the count falls out of the viewport
+    // rather than being chosen, and NOTHING measured it. The card's own verify says it: "perRow is
+    // currently printed nowhere and asserted nowhere. A number printed but not asserted on is
+    // decoration."
+    // ⚠ REPORTED, NOT YET ASSERTED, and that is deliberate. The desktop target is settled (~4) but
+    // the MOBILE one is not: 3 per row at 390px is a ~120px card, which cannot hold the wrapped
+    // print footer this same gate asserts is readable. Asserting a number the Driver has not
+    // ruled on would encode my taste as a requirement. Measuring it first is what makes the
+    // ruling answerable — and it is the precondition for tuning, since you cannot tune toward a
+    // target you are not measuring.
+    // Row-mates are found by TOP EDGE, not by index: a grid row is a set of boxes sharing a y,
+    // and counting "every Nth card" assumes the very layout under question.
+    // ⚠ IT IS THE MAX, AND THE WALL IS SECTIONED. Grouping every card by top edge at 1440px gives
+    // row sizes [5,5,5,3,3,2,2,1,1] — the short rows are the REMAINDERS of collapsible sections,
+    // not a density failure. Reading the first row alone would be right here by luck and wrong on
+    // a wall whose first section holds one poster. The max across all rows is the density the
+    // Driver's "around 4 per row" is about; cardW is reported beside it because the two move
+    // together and the width is what explains the count.
+    let perRow = 0, cardW = 0
+    if (cards.length) {
+      const byTop = new Map()
+      for (const c of cards) {
+        const t = Math.round(c.getBoundingClientRect().top / 8) * 8
+        byTop.set(t, (byTop.get(t) || 0) + 1)
+      }
+      perRow = Math.max(...byTop.values())
+      cardW = Math.round(cards[0].getBoundingClientRect().width)
+    }
     return { cards: cards.length, rows: cards.filter((c) => c.querySelector('.pfs')).length,
              overflowing, worst, intrusions, coveringPreview, worstY,
              previewsPainted, previewsBroken, noPreview, clipped: clipped.length,
-             controls: ctrls.length, tiny: tiny.length }
+             controls: ctrls.length, tiny: tiny.length, perRow, cardW }
   })()`)
   await page.close()
   return { width, lang, hover, reach, ...out }
@@ -247,6 +277,7 @@ for (const r of results) {
     + ` · over-preview ${r.coveringPreview}${r.worstY ? ` (${r.worstY}px)` : ''}`
     + ` · previews ${r.previewsPainted} painted`
     + `${r.previewsBroken ? `/${r.previewsBroken} BROKEN` : ''}${r.noPreview ? `/${r.noPreview} none` : ''}`
+    + ` · max ${r.perRow}/row @ ${r.cardW}px`
     + ` · ${r.controls} control(s), ${r.tiny} too small, ${r.clipped} clipped`
     + (r.reach ? ` · last card ${r.reach.lastInView ? 'reachable' : 'UNREACHABLE'}` : '')
     + (r.hover