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.

AGENTS.md

@@ -191,6 +191,8 @@ Ely GPUI Component. A component library for GPUI, in light and dark.
 - The chart's hues go by `theme::HUE_NAMES`, for an event's color and a label's alike.
 - Text whose every character counts, encoded data and keys, goes through `typography::literal`: JetBrains Mono draws `==` and `->` as ligatures (`devtools::Encoder`).
 - Messages go through `i18n::I18n`, a global the app sets: `cx.global::<I18n>().text(key, args)` fills each `{name}`. Every catalog holds the first one's keys; a missing key, argument or catalog fails. A right-to-left locale mirrors a stack with `Direction::row` and sets its text with `Direction::align`; CoreText orders mixed Hebrew and Latin runs on macOS, but gpui sets no paragraph direction, so a line takes its first strong character's: `text` leads a right-to-left message with U+200F, so its words suit labels, not a field's value, and `Locale::number` a right-to-left minus with U+200E, as CLDR does. gpui 0.2.2 wraps shaped glyphs in the order they are drawn, so a right-to-left line that wraps comes out garbled: right-to-left text stays on one line, an `Ellipsis`, which cuts by characters and sits by its text's alignment.
+- gpui orders Tab stops by `tab_index` before paint order, and registers none for an `invisible()` element. A stop that must come first where it sits, yet draw over what paints after it, stays in flow at zero size and raises what it shows (`i18n::SkipLink`): a lower `tab_index` would sort it before the root scope's own handle, so Tab from the root would pass it.
+- Color-blind safe charts swap 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 (`Theme::set_color_blind_safe`, in Ely's own palettes alone).
 
 ## Decisions