Give Preview checkboxes their pointer-free path via the system focus model

10 promised toggleable-without-pointer, 11's normative inventory had the
toggle as pointer-only, and 02's lock disabled it "via menu validation"
with no menu item to validate. Settled: checkboxes and links are real
controls in the focus/accessibility tree — FKA + Space and VO-Space toggle
them; in-content controls are content, not commands, so 04's menu-item
contract and 11's existence rule scope to commands; under the read-only
lock the controls disable in place.

Claude-Session: https://claude.ai/code/session_01HJ7PhFNmQ19bvy9RMD6GSb

DESIGN/11-command-nexus.md

@@ -76,7 +76,7 @@ All board grammar keys are inert while a title editor is focused, and menu dispa
 - **Drag & drop — the locality model** (04): within-board move / cross-board copy; **⌥ always forces copy, ⌘ always forces move**; multi-drag; lane header is the lane drag surface; drag-to-restore from the trash.
 - **Lane edge drag** (03 ▸ Lane): window-growing resize between integer widths — the one width control that moves the window.
 - **Finder file drops** (04): onto a card = attach; onto lane empty space = one card per file; anywhere on the card window = attach (05's payload-split precedence).
-- **Preview** (05): task-list checkbox toggle (the one interactive exception), link opens, text selection; carousel paging on the card face (03).
+- **Preview** (05): task-list checkbox toggle (the one interactive exception), link opens, text selection; carousel paging on the card face (03). **In-content controls aren't P-only** (settled): checkboxes and links are real controls in the focus/accessibility tree, so Full Keyboard Access + Space and VO-Space reach them (05 ▸ Task-list checkboxes) — content rides the system focus model rather than earning command rows; "a command absent here doesn't exist" scopes to commands, not content.
 - **Attachment rows** drag out their file URL (05). **Welcome rows**: single click selects, double click opens (03).
 
 ## Context menus