Add complete keyboard navigation.

specs/layout.allium

@@ -72,6 +72,26 @@ surface AppShellSurface {
         shell.sidebar.width
         shell.content_area.panel.visible
         shell.assistant_sidebar.visible
+
+    @guarantee KeyboardTraversal
+        -- Tab and Shift+Tab traverse every enabled button, checkbox, picker,
+        -- and text editor in visual order. Disabled controls are skipped.
+        -- Enter or Space activates the focused control.
+
+    @guarantee KeyboardHints
+        -- Holding Control+Option on macOS or Control+Alt on Linux/Windows reveals
+        -- an access-key marker on every enabled control intersecting the viewport.
+        -- Semantic controls use their direct one-character shortcut. Remaining
+        -- visible controls receive stable two- or three-letter A-Z codes in
+        -- widget-tree order, excluding direct letters as generic prefixes.
+        -- Every marker is a warm-yellow box with black text and border, placed
+        -- wholly inside its control without obscuring activity-bar icons.
+        -- Typing a prefix while holding the modifier hides non-matches and shows
+        -- only each candidate's remaining suffix; a complete code activates its
+        -- control. Escape or modifier release cancels the access-key mode.
+        -- Covered layers and off-screen scroll content receive no code. A focused
+        -- Option/Alt alone always remains available for character input, and a
+        -- focused text editor retains modified input instead of entering the mode.
 }
 
 value ContentArea {
@@ -162,6 +182,7 @@ value ActivityButton {
     label_key: String                -- i18n key for tooltip
     badge: Badge?                    -- only git has a badge
     active: Boolean                  -- highlighted when this view is showing
+    access_key: String               -- Control+Option (macOS) or Control+Alt + 1..9,0 in displayed order
 }
 
 value Badge {
@@ -317,3 +338,9 @@ value TokenUsage {
 
 -- Ctrl/Cmd+B: toggle sidebar
 -- Ctrl/Cmd+W: close active tab (see tabs.allium)
+-- Tab / Shift+Tab: next / previous enabled control
+-- Enter / Space: activate focused control
+-- Control+Option (macOS) or Control+Alt + 1..9,0: activate the corresponding activity
+-- Post editor Control+Option (macOS) or Control+Alt + M/X: toggle metadata/excerpt
+-- Import and Settings sections expose unique Control+Option or Control+Alt access keys
+-- Site Validation uses R/A and Metadata Diff uses R with the platform modifier pair