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/AgentGuide.swift

@@ -96,7 +96,13 @@ enum AgentGuide {
     /// lanes by editing frontmatter — and, more to the point, has to know that `collapsed: true` is why
     /// a lane it wrote a card into is not showing it. One clause beside `width` in Frontmatter, with the
     /// remove-to-expand rule stated because writing `false` is the mistake the key invites.
-    static let version = 12
+    /// **v13 names the card's `hero` key** (03-board-ui.md § Card face ▸ Hero image): a card face draws
+    /// one of the card's own attachments as a banner, and since there is no in-app setter this version,
+    /// an agent writing the key *is* how a hero image gets set. One clause beside the other card keys
+    /// in Frontmatter, spelling the grammar the reading enforces — a bare filename, never a path —
+    /// because a path is exactly what an agent that has just written `![](attachments/x.png)` into a
+    /// body will reach for.
+    static let version = 13
 
     // MARK: - The version marker
 
@@ -409,6 +415,16 @@ enum AgentGuide {
     absent key is the default, and the app removes it too. A lane's `width`
     rides along untouched while it is folded.
 
+    Cards may set `hero` — **the bare filename of one of that card's own
+    attachments** (`hero: sketch.png`), which the app draws as a banner across
+    the top of the card's face. **A filename, never a path**: `attachments/` is
+    implied, so `hero: attachments/sketch.png` and any other value containing a
+    `/` name nothing and draw nothing. The file has to sit directly in that
+    card's `attachments/` folder (Attachments below); a name that is missing,
+    unreadable or not an image draws no banner and is otherwise harmless, so a
+    hero set before the file arrives simply starts working when it does. There
+    is no control for this in the app — writing the key is how a hero gets set.
+
     **Quote any `title` containing a colon** — `title: Fix: the thing` is
     invalid YAML; write `title: "Fix: the thing"`. The same goes for any value
     containing `: ` or starting with `#`, `[`, `{`, or a quote — when in doubt,
@@ -548,7 +564,9 @@ enum AgentGuide {
       If the name is taken, pick a free one Finder-style (`shot.png` →
       `shot 2.png`) — never overwrite.
     - Reference attachments from the card body by relative path:
-      `![](attachments/sketch.png)`.
+      `![](attachments/sketch.png)`. To put that same picture on the card's
+      face, add `hero: sketch.png` to the frontmatter — the bare name, without
+      the folder (Frontmatter above).
     - Subfolders under `attachments/` are tolerated but the app never creates
       or lists them — keep attachments top-level.