Hero image for cards — one of the card's own attachments, banded across its face

A card whose `hero:` names one of its own attachments draws that picture as a
banner across the full width of its plate, above the icon-and-title row,
aspect-fill cropped into a fixed 2.75 em band — 36pt at the standard body, and
em-scaled like every other figure the board draws, so it grows with the system
text size and with the board's zoom rather than shrinking against a title twice
its usual size. The figure sits deliberately under the 44pt a plain one-line
card is tall: a hero card should read as a card with a picture on it rather than
a picture with a caption, which is 03's standing rule that the title dominates.

The key's grammar is a **bare filename**, and that is what separates it from the
board background's `image` subkey rather than a nervousness about paths. A board
names a file anywhere under its root, so a path is that key's reading and where
it leads is the renderer's question. A card names one of the files it already
owns — the flat `attachments/` folder the app lists, relocates into, and carries
through every move, copy, trash and restore — so `hero: art/sketch.png` is not an
awkward spelling of a hero image, it is a value the key cannot mean. It therefore
has no reading at all: a value carrying a separator, or spelling `.`/`..`, or
empty, is malformed at the document layer, which renders it as absent and leaves
the coerce tier's trace, exactly as `width: 1.5` does. The bytes stay as written,
the resolver re-checks containment anyway, and the whole degrade family below
that — a name pointing at a missing file, an unreadable one, or one that is not
an image — ends the same way: no banner, no defect, nothing written.

That last promise is about *height* as much as about ink, so the band is given no
height at all until a picture has actually decoded. A card whose hero cannot be
drawn lays out identically to a card with no key, structurally rather than by a
branch somebody has to remember; the price is one settle per hero as a board
opens, and none after that. Everything else the face draws is attached outside
the new stack and is untouched by it — the accent stripe still runs the plate's
full leading edge across the band's corner, the selection and file-hover strokes
still ring the whole plate, the cut and drag dims still cover it, and the drop
model still registers the plate's real height, so a hero card is simply a taller
card the masonry already understands. The trash draws it too, by the one-face
rule.

Decoding is ImageIO's downsampling path off the main actor at a quarter of the
backdrop's pixel budget (`BoardBackdrop.decode` gained the limit as a parameter
rather than being copied), and the results live in one app-wide, deliberately
non-observable cache keyed on path plus the file's date and size. Non-observable
because a tracked write there would invalidate every hero face on the board,
which is the O(board) invalidation this view was rebuilt once already to shed;
each face holds its own picture in view state and seeds it from the cache, which
is also what lets the drag replica — whose preview builder is non-escaping and
cannot await anything — carry the band at the face's real height. Taking a stamp
twice from one URL value turned out to answer with the first read's date and size
however many times the bytes had changed, so `stamp(of:)` now drops its cached
resource values first; noticing a replacement is the only thing a stamp is for.

The face takes the resolved URL as a compared input rather than resolving it, for
selected-ness's reason one axis over: resolving needs the card's folder, which a
face does not know, and finding it from the snapshot would be a board walk per
face. The lane and the trash column each know their own container and compute it
once for the whole strip.

There is no in-app setter this version — the key is written by hand or by an
agent, which is why the guide bumps to v13 with a clause spelling the grammar out
beside the other card keys, and why `attachments/` gets the one-line pointer an
agent that has just written `![](attachments/x.png)` will need. "Set as Hero"
from the attachment row is future work, as is the card window and print, which
draw the same model and show no banner today.

Claude-Session: https://claude.ai/code/session_014PtZdPwqZuqEDLc6wZMtEy

KanbanTests/BoardLoaderTests.swift

@@ -1065,6 +1065,43 @@ struct BoardLoaderOptionalOrderTests {
         #expect(byID[ranked] == 1024)
         #expect(byID[orderless] == 2048)
     }
+
+    /// **`hero` rides onto the card model, on both sides of the container boundary** (03-board-ui.md
+    /// § Card face ▸ Hero image). The trash half is the one worth writing down: a trashed card is an
+    /// ordinary card in a special place and draws the same face, so a snapshot that dropped the key
+    /// on the way through `.trash/` would silently un-band every deleted card.
+    ///
+    /// The malformed case rides too, as a *shape* rather than a value: the coerce tier reports on it
+    /// (`coercedFrontmatter`), and the face reads it as no hero at all.
+    @Test func heroCarriesOntoCardsInBothContainers() throws {
+        let fixture = try BoardFixture()
+        defer { fixture.tearDown() }
+
+        let lane = uuidFolderName()
+        let card = uuidFolderName()
+        let pathy = uuidFolderName()
+        let bare = uuidFolderName()
+        let trashed = uuidFolderName()
+
+        try fixture.index("", "schema: 1\n")
+        try fixture.index(lane, "schema: 1\norder: 1024\n")
+        try fixture.index("\(lane)/\(card)", "schema: 1\nkind: card\norder: 1024\nhero: sketch.png\n")
+        try fixture.index("\(lane)/\(pathy)", "schema: 1\nkind: card\norder: 2048\nhero: art/sketch.png\n")
+        try fixture.index("\(lane)/\(bare)", "schema: 1\nkind: card\norder: 3072\n")
+        try fixture.index(".trash/\(trashed)", "schema: 1\nkind: card\nhero: cover.jpg\n")
+
+        let result = try BoardLoader.load(boardRoot: fixture.root)
+        let cards = Dictionary(uniqueKeysWithValues: result.model.lanes[0].cards.map { ($0.id.rawValue, $0.hero) })
+        #expect(cards[card] == .valid("sketch.png"))
+        #expect(cards[pathy] == .malformed(raw: "art/sketch.png"))
+        #expect(cards[bare] == .missing)
+        #expect(result.model.trash.map(\.hero) == [.valid("cover.jpg")])
+
+        let byPath = Dictionary(uniqueKeysWithValues:
+            result.coercedFrontmatter.map { ($0.path, $0.fields.map(\.key)) })
+        #expect(byPath["\(lane)/\(pathy)/index.md"] == ["hero"])
+        #expect(byPath["\(lane)/\(card)/index.md"] == nil)
+    }
 }
 
 // MARK: - Encoding strictness