a11y: a skip link, color-blind safe charts

i18n::SkipLink holds a Tab stop in flow at zero size, first where it
sits, and while focused raises a pill over the page; Enter, Space or a
press hands focus to its target. gpui orders stops by tab_index before
paint and registers none for an invisible element, so the stop stays in
flow and only the pill is raised. Theme::set_color_blind_safe swaps Ely's
chart hues for a set fitted to stay apart under Machado's protanopia,
deuteranopia and tritanopia; a test holds every pair 0.10 apart in Oklab
and each hue 3:1 on its page, and red against green shows the measure
bites. KeyboardNavigationHelper points to interaction::RovingFocus. The
announcer, labels and screen-reader text stay blocked: gpui 0.2.2 has no
accessibility tree, and nothing here is trusted to observe VoiceOver.
The i18n tests move under src/i18n/tests.

tasks/ch31-43.md

@@ -280,13 +280,13 @@
 
 ## T40b a11y — `src/i18n`
 
-- [ ] LiveRegion / Announcer — blocked: gpui 0.2.2 has no accessibility tree
-- [ ] SkipLink
+- [ ] LiveRegion / Announcer — blocked: gpui 0.2.2 has no accessibility tree; AppKit's announcement request needs none, but no process here is trusted to observe VoiceOver take it
+- [x] SkipLink → `i18n::SkipLink`: a Tab stop in flow that shows only while focused, raised over the page, and hands focus to its target
 - [ ] AccessibleIcon — blocked: no accessibility tree
 - [ ] AriaLabel helper — blocked: no accessibility tree
-- [ ] KeyboardNavigationHelper
+- [x] KeyboardNavigationHelper → `interaction::RovingFocus`
 - [ ] ScreenReaderOnly — blocked: no accessibility tree
-- [ ] ColorBlindSafe Palette
+- [x] ColorBlindSafe Palette → `Theme::set_color_blind_safe`: chart hues fitted to stay apart under Machado's simulated protanopia, deuteranopia and tritanopia, each 3:1 on its page
 
 ## T41 Maps — `src/maps`