Build Preview mode rendering

The card body's resting state: swift-markdown (pinned 0.8.0, smart
typography off — Preview renders the bytes on disk) parsed into a pure
BodyMarkup model with UTF-8 source offsets, rendered on one hosted
TextKit 1 NSTextView — chosen because find-in-text is NSTextFinder,
checkbox clicks reuse AppKit character hit-testing, links are .link
attributes, and NSTextTable's automatic layout is exactly the
columns-sized-to-contents rule. The GFM subset renders per 05; HTML
stays verbatim code-styled text; relative images resolve against the
card folder while remote URLs are never fetched, drawing a quiet chip
instead. Task checkboxes are live: a click flips exactly one byte
through a fresh-read, refuse-uneditable, stamp, atomic-replace write —
the app's only offset-addressed write, so a moved target refuses as
staleTarget and what the user saw decides the direction, netting one
toggle on a double-click. Empty bodies open in Edit per CardBodyMode's
opening rule, applied once; the Edit surface itself stays an honest
read-only stub until its card. FindCommand prefers the card body's
find over board search when a card window is focused.

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

KanbanTests/CardWindowShellTests.swift

@@ -189,6 +189,68 @@ struct CardWindowMetricsTests {
     }
 }
 
+// MARK: - The body column's mode
+
+/// 05-card-window.md ▸ Mode grammar, as the two rules a unit test can hold: **Preview is the resting
+/// state unless the body is empty**, and **the flip is one flip** whichever key produced it.
+///
+/// Both are silent failures of exactly the kind this file exists for. A card that opened in Preview
+/// with an empty body would look like a working window showing nothing, and the user would have to
+/// discover ⌘E to write the first word of a card they just made — which is the ceremony the rule was
+/// settled to remove ("a new card has nothing to preview, so ⌘↩ during creation flows title → body
+/// without a mode stop").
+@MainActor
+@Suite("Card body mode")
+struct CardBodyModeTests {
+
+    @Test("A card with a body opens in Preview")
+    func aCardWithABodyOpensInPreview() {
+        #expect(CardBodyMode.opening(body: "# Notes\n") == .preview)
+        #expect(CardBodyMode.opening(body: "x") == .preview)
+    }
+
+    @Test("An empty body opens straight into Edit — whitespace included")
+    func anEmptyBodyOpensInEdit() {
+        #expect(CardBodyMode.opening(body: "") == .edit)
+        #expect(CardBodyMode.opening(body: "\n") == .edit)
+        // A card the app itself just minted has exactly this body: `BoardWriter.newDocumentText`
+        // writes frontmatter and nothing after the closing delimiter.
+        #expect(CardBodyMode.opening(body: "   \n\t\n") == .edit)
+    }
+
+    @Test("⌘E, Return in Preview and Escape in Edit are one flip")
+    func theToggleIsSymmetric() {
+        #expect(CardBodyMode.preview.toggled == .edit)
+        #expect(CardBodyMode.edit.toggled == .preview)
+        #expect(CardBodyMode.preview.toggled.toggled == .preview)
+    }
+
+    @Test("The opening rule runs once, not on every snapshot")
+    func theOpeningRuleIsAppliedOnce() {
+        let presentation = CardBodyPresentation()
+        #expect(presentation.openIfNeeded(body: "# Notes\n") == .preview)
+
+        // The user presses ⌘E …
+        presentation.toggleMode()
+        #expect(presentation.mode == .edit)
+
+        // … and a watcher reload arrives with the same body. Re-deciding here would throw the user
+        // out of the surface they just asked for.
+        #expect(presentation.openIfNeeded(body: "# Notes\n") == .edit)
+        #expect(presentation.mode == .edit)
+    }
+
+    @Test("A window that opened into Edit is not dragged back to Preview when the body fills up")
+    func aLaterBodyDoesNotReopenTheRule() {
+        let presentation = CardBodyPresentation()
+        #expect(presentation.openIfNeeded(body: "") == .edit)
+
+        // The user types; the reload brings the text back. The rule is about *opening* a card, and
+        // this window is already open.
+        #expect(presentation.openIfNeeded(body: "first words") == .edit)
+    }
+}
+
 // MARK: - Identity and frames
 
 @MainActor