feat(headless): add Tabs primitive

[alexcarpenter]

Summary

• Adds Tabs primitive with animated indicator via CSS custom properties
• Roving tabindex navigation, manual/automatic activation modes
• shouldForceMount for directional slide animations

Review Stack

Review order — start at #8474 (foundation) and work up. Each PR adds one primitive on top of the previous.

#	PR	What it adds	Base
1	#8474	Foundation (infra + hooks + utils + Dialog)	main
2	#8475	Accordion	← #8474
3	#8476	Tabs	← #8475
4	#8477	cssVars + Tooltip	← #8476
5	#8478	Popover	← #8477
6	#8479	Select	← #8478
7	#8480	Menu	← #8479
8	#8481	Autocomplete + integration tests	← #8480

Test plan

• pnpm build succeeds
• pnpm test passes (+45 tabs tests)
• CI passes

🤖 Generated with Claude Code

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Project	Deployment	Actions	Updated (UTC)
clerk-js-sandbox	Ready	Preview, Comment	May 5, 2026 6:39pm

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