perf(web): Shift re-renders only each row's quick-action group#22726

Tapping Shift on the pull requests list cost about 15k renders. speedMode was a row prop, so every Shift press re-rendered every row along with its tooltips, buttons and icons, only to toggle display on the quick-action group.

Shift now reaches the rows through PullRequestSpeedModeContext. PullRequestListGroups provides it (PullRequestSpeedModeProvider holds the only key subscription), and a small SpeedModeGroup wrapper inside PullRequestSpeedActions is the only consumer. Rows are memo, so they bail out; the buttons are the wrapper's children, so they bail out too. Only the wrapper re-renders, one per row with quick actions. The thread panel's pull request list provides the same context from the Shift state it already tracks, so it behaves as before.

I first tried a useShortcutModifierState call in each wrapper. It cut renders the same way but raised commits from ~20 to ~820. Each row's window listener runs in its own task, so React committed once per row. The single provider keeps it to one commit per key change.

Shift tapped 3 times, 136 rows listed (rc-init render counter, dev server seeded from real data, fresh browser per side, base 6a6c84f990 vs this layer):

Run 1 on both sides includes rows refreshing as the list data settles after load. After the change, the Shift renders are 816 SpeedModeGroup (136 rows × 6 key transitions) plus the sidebar, which listens to modifiers on its own.

Tests run: tsc --noEmit for apps/web, vp lint on the changed files, vp test run src/reactCompilerCoverage.test.ts -t pullRequest (0 bailouts).

Part of the pull request page split stack: #22671 → #22672 → #22702 → #22703 → #22704 → #22705 → #22706 → #22726 → #22727.

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