[tabs] Snap indicator to device pixels

[atomiks]

Tabs indicators could inherit sub-device-pixel edges from fractional layout measurements, which made border-aligned indicators look slightly blurry. (Noticed in #4676) This snaps hydrated interior active-tab edges to the device pixel grid while preserving boundary edges that need to stay flush with the tab list or an adjacent panel border.

Changes

• Snap interior active-tab edges with the device pixel ratio in hydrated positioning.
• Keep first/last tab boundary edges flush with the tab list so bordered indicators can connect to panel borders.
• Keep the inline prehydration script lightweight and restore its existing fallback dimension behavior.
• Add Chromium coverage for fractional horizontal and vertical tab edges, boundary-edge alignment, and the cross-axis scrollbar regression.

[pkg-pr-new]

• base-ui-tanstack-start

• vite-css-base-ui-example

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/react@4716
  

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/utils@4716
  

commit: a3b41d0

[code-infra-dashboard]

Bundle size

Bundle	Parsed size	Gzip size
@base-ui/react	🔺+243B(+0.05%)	🔺+108B(+0.07%)

Details of bundle changes

Performance

Total duration: 1,248.43 ms 🔺+61.47 ms^(+5.2%) | Renders: 53 ⁽⁺⁰⁾ | Paint: 1,908.70 ms 🔺+74.21 ms^(+4.0%)

Test	Duration	Renders
Checkbox mount (500 instances)	93.58 ms 🔺+28.33 ms(+43.4%)	1 (+0)
Select mount (200 instances)	142.34 ms 🔺+14.53 ms(+11.4%)	3 (+0)
Select open (500 options)	56.48 ms 🔺+7.16 ms(+14.5%)	14 (+0)
Popover mount (300 instances)	93.37 ms 🔺+5.19 ms(+5.9%)	2 (+0)
Dialog mount (300 instances)	78.46 ms 🔺+4.95 ms(+6.7%)	2 (+0)

…and 7 more — details

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Check out the code infra dashboard for more information about this PR.

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