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

Kanban/Storage/FrontmatterFields.swift

@@ -116,6 +116,7 @@ extension FrontmatterDocument {
         record(FrontmatterKeys.background, backgroundImage)
         record(FrontmatterKeys.icon, icon)
         record(FrontmatterKeys.iconColor, iconColor)
+        record(FrontmatterKeys.hero, hero)
         record(FrontmatterKeys.kind, kind)
         return found
     }
@@ -241,6 +242,34 @@ extension FrontmatterDocument {
         }
     }
 
+    /// **A card's hero image — one of its own attachments, by bare filename**
+    /// (03-board-ui.md § Card face ▸ Hero image; `FrontmatterKeys.hero`).
+    ///
+    /// The string family's scalar coercion with one gate after it: the reading has to be a name a
+    /// card's `attachments/` folder could hold. A value carrying a path separator, or spelling one of
+    /// the two directory entries every folder has (`.`, `..`), or empty, is **malformed** — it renders
+    /// as no banner and leaves the coerce tier's trace, exactly as `width: 1.5` does.
+    ///
+    /// **Deliberately stricter than `backgroundImage`, and the difference is grammar rather than
+    /// caution.** A board's `image` subkey names a file anywhere under the board root, so a path *is*
+    /// its reading and where it leads is the renderer's question. `hero` names one of the card's own
+    /// attachments — the flat, top-level files 01-storage-format.md § Attachments already defines and
+    /// that travel with the card through every move and copy — so `hero: art/sketch.png` is not an
+    /// awkward spelling of a hero image, it is a value naming something the key cannot mean. Refusing
+    /// it here is what makes "no paths, no URLs" a fact of the schema rather than a convention the
+    /// resolver happens to enforce; the resolver re-checks containment anyway (`CardHero`), because a
+    /// lenient reading must never be the only thing standing between a value and the filesystem.
+    ///
+    /// A file that is missing, unreadable, or not an image is **not** this layer's business and is
+    /// perfectly `.valid` here: those are facts about the disk, and they degrade at the face — no
+    /// banner, no defect, bytes untouched (`CardHeroImage`).
+    public var hero: FieldValue<String> {
+        read(FrontmatterKeys.hero) { value, raw in
+            guard let text = Self.string(value, raw: raw), Self.isBareFilename(text) else { return nil }
+            return text
+        }
+    }
+
     public var created: FieldValue<Date> { read(FrontmatterKeys.created) { value, _ in Self.date(value) } }
     public var modified: FieldValue<Date> { read(FrontmatterKeys.modified) { value, _ in Self.date(value) } }
     public var deleted: FieldValue<Date> { read(FrontmatterKeys.deleted) { value, _ in Self.date(value) } }
@@ -326,6 +355,19 @@ extension FrontmatterDocument {
         return value >= 1 ? Int(value) : 1
     }
 
+    /// Whether `text` is a name one folder could hold — the gate `hero` puts after the string
+    /// coercion (see it for why the strictness lives at this layer).
+    ///
+    /// Three refusals, and nothing else: a **path** (any `/`, wherever it sits — leading, trailing or
+    /// interior, which covers absolute paths, subfolder paths and climbs out with `..` in one rule),
+    /// the two **directory entries** every folder carries, and the **empty** string. A name is not
+    /// checked for anything else — leading dots, spaces, `~`, and characters the author's filesystem
+    /// may or may not accept are all a filename's business, and a name that names nothing simply
+    /// resolves to nothing later.
+    private static func isBareFilename(_ text: String) -> Bool {
+        !text.isEmpty && !text.contains("/") && text != "." && text != ".."
+    }
+
     /// YAML 1.1's boolean words, for a **quoted** scalar that spells one — the coercion `collapsed`
     /// needs and no other field wants. Case-insensitive, because the grammar itself is (`True`, `ON`,
     /// `Yes` are all booleans unquoted); anything else has no boolean reading.