fix(Tabs): keep the tab selection controlled from the first render

[mfal]

Part of #3014 — the A component changed from uncontrolled to controlled half. Split out of #3025, which carries the PressResponder half. Neither PR closes the issue on its own; close it once both are merged.

The warning (14×) — all of them Tabs

Not SegmentedControl, as the issue guessed: the two pages that looked like SegmentedControl cases both happen to contain a Tabs example, as does detail-page.

Tabs passed selectedKeyProps ?? selectedKeyState to Aria.Tabs. With neither selectedKey nor defaultSelectedKey given that is undefined on the first render, so Aria.Tabs starts uncontrolled. The tab titles reach Aria.TabList through a tunnel and commit one paint later, so react-aria's collection is still empty on render 1 and only picks the default tab in the next commit. That selection lands in selectedKeyState, the following render passes a defined key, and useControlledState warns.

Fix

?? null. useControlledState treats only undefined as uncontrolled, so null reads as "controlled, nothing selected yet" and the auto-selection behaves exactly as before. Aria.TabsProps narrows selectedKey to Key while the state hook behind it accepts null — one documented cast bridges that upstream type gap.

Verified

• Reproduced first against the real stories with captureOwnerStack() to name the culprit.
• Red-green: reverting ?? null fails the added test.
• Behavioural assertions, not warning-absence: the first tab is selected on mount, and clicking a tab shows its panel.
• Story sweep after the fix — Tabs/Default, Tabs/Collapsed, Tabs/TabNotFound, SegmentedControl/Default: zero occurrences of the warning.
• No rendered output changes, so no visual baseline moves.

🤖 Generated with Claude Code

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Coverage Report for ./packages/components/

Status	Category	Percentage	Covered / Total
🔵	Lines	78.69%	746 / 948
🔵	Statements	78.57%	763 / 971
🔵	Functions	80.09%	165 / 206
🔵	Branches	70.33%	377 / 536

File Coverage

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Generated in workflow #6567 for commit df1009f by the Vitest Coverage Report Action

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