Build the Raw Source outlet

The escape hatch: View > Raw Source (opt-cmd-E) unmounts the whole
content area for the literal on-disk index.md in a plain monospaced
editor with Cancel/Apply. Raw source is window-level state, not a third
body mode — entry rides setMode(.preview), which flushes the Edit
session by construction, then reads the file fresh; exit reveals
Preview, and an empty body after Apply does not reopen Edit (openIfNeeded
already ran). Apply validates the proposed bytes through the loader's
own card checks — parseDocument's strict UTF-8/BOM rejection, schema,
order — deliberately skipping the uneditable-shape refusal, since a
flow-mapping card is exactly what the hatch repairs; invalid bytes
alert in place with the loader's own error and no bracket opens. The
write is byte-for-byte with no modified stamp and no modified-by clear,
per 01's explicit carve-out — the verbatim contract outranks stamping —
and identical bytes write nothing. Escape cancels, cmd-Return applies,
toggle-off applies too, and cmd-E disables while raw is active via a
testable predicate. Tombstoned targets refuse as vanished: a foreign
delete is never reverted by a stale buffer.

Claude-Session: https://claude.ai/code/session_01SR4XGjmBE16ZUYWpfFHXwY

Kanban/LiveStore/BannerCenter.swift

@@ -652,6 +652,13 @@ public final class BannerCenter {
             // act happening on its own. The keystrokes are still in the buffer — the banner says the
             // app could not put them on disk, not that they are gone.
             if let title { "Couldn't save '\(title)'" } else { "Couldn't save the card" }
+        case let .rawSource(title):
+            // **Apply**, because that is the button they pressed (05-card-window.md ▸ Raw source
+            // outlet), and "source changes" because what failed to land is the whole file as they
+            // typed it — not a save of the card's body, which is what "Couldn't save" would claim.
+            // The buffer is still on screen: the banner says the app could not put those bytes on
+            // disk, not that they are gone.
+            if let title { "Couldn't apply source changes to '\(title)'" } else { "Couldn't apply the source changes" }
         }
     }
 
@@ -668,6 +675,12 @@ public final class BannerCenter {
             message
         case let .staleTarget(message):
             message
+        case let .invalidSource(error):
+            // The loader's own reason, without its path: the path is always the card's own
+            // `index.md`, and the banner has already named the card. In practice the raw-source
+            // outlet raises this in its alert and never here — the store validates before it opens a
+            // write bracket — so this line exists for a caller that reached the Writer directly.
+            error.reason.description
         }
         return trimmed(text)
     }