Port · Parity: every page matches gpui 0.2.2's; the rules re-tested

Every page and scripted state was shot with gpui 0.2.2 and with the
port, light and dark, full width and at 280px, judged pair by pair by
agents with skeptics, and every page's log diffed. What differed is
fixed, each with a test that fails without it where the test platform
can show it:

- The gallery's harness takes posted mouse moves (gpui_macos listens
  to a tracking area), posts each drag move twice (it no longer
  replays a held drag), and waits three frames before each shot.
- Rolling digits: a block column, so taffy 0.13 lends the cell no
  baseline, and a line of whole device pixels, since gpui snaps each
  length on its own. DescriptionList's row aligns ends.
- Margins that taffy 0.13 collapses through a plain block: the
  drawer's pull and snap, the stagger and transition rise, the dialog
  and palette rises, the fullscreen dialog's slide.
- An Input keeps room to type in a squeezed row; the field rows drop
  w_full, whose reason was taffy 0.9's.

AGENTS.md's rules that named gpui 0.2.2 or taffy 0.9 were re-tested
against both sources and rewritten, and new ones say what the port
changed. TASKS.md's tags follow: what the new gpui makes possible is
prove. Captions, doc comments and README say what holds now.

tasks/ch01-10.md

@@ -17,7 +17,7 @@
 - [x] Show / When — native `.when()` / `.when_some()`
 - [x] For / Each — native `.children(iter)`
 - [x] Fragment — native: children take iterators
-- [x] VisuallyHidden — blocked: gpui 0.2.2 has no accessibility tree
+- [ ] VisuallyHidden — prove: gpui builds an AccessKit tree on macOS, Windows and Linux, not the web, from elements with an id and a `role`; a box that draws nothing, with an `aria_label`, must reach the macOS tree with its label
 - [x] FocusRing
 - [x] FocusScope / FocusTrap
 - [x] ClickOutside — native `on_mouse_down_out`
@@ -129,7 +129,7 @@
 - [x] TrayIcon / TrayMenu — macOS `NSStatusItem`; other systems get an error
 - [x] DockBadge — macOS dock tile; other systems get an error
 - [x] JumpList — native `cx.set_dock_menu()`
-- [x] SystemNotification — blocked: an unbundled app has no notification center; gpui 0.2.2 has no API
+- [ ] SystemNotification — prove: on macOS `cx.show_system_notification` posts only from an app bundle; unbundled, as `cargo run` starts it, gpui logs and drops the call, so Ely must check the bundle identifier and fail loud. A bundled gallery must post one and hand its response to the owner through `on_system_notification_response`
 - [x] MultiWindow Manager
 - [x] WindowSwitcher
 - [x] QuickLauncher → `navigation::QuickLauncher` (T07c): a SearchPalette in an always-on-top window
@@ -205,7 +205,7 @@
 - [x] RadioCard
 - [x] Switch / Toggle
 - [x] Select
-- [x] NativeSelect — blocked: gpui 0.2.2 has no native popup menu
+- [x] NativeSelect — blocked: gpui has no native popup menu
 - [x] Combobox
 - [x] Autocomplete → `forms::Combobox::free`
 - [x] MultiSelect