Switch Turbo progress bar animation from width to compositor-friendly transform: scaleX(...)

[Copilot]

Turbo’s progress bar currently animates width, which incurs layout/paint work while the main thread is busiest. This change keeps the same visual behavior (10% start, trickle to 100%, fade on hide) but moves progress interpolation to transform, allowing compositor-thread animation.

• Progress bar rendering path (src/core/drive/progress_bar.ts)

  • Set .turbo-progress-bar to width: 100% and animate transform instead of width.
  • Added transform-origin: 0 0, transform: scaleX(0) translateZ(0), and will-change: transform, opacity.
  • Updated install/reset path from style.width = "0" to style.transform = "scaleX(0) translateZ(0)".
  • Updated refresh() to compute scale from existing progress semantics and apply scaleX(...).

• Progress bar assertions in functional tests

  • Updated progress-bar tests in:
    • src/tests/functional/navigation_tests.ts
    • src/tests/functional/form_submission_tests.ts
  • Added assertions that the progress bar uses style.transform (scaleX(...) translateZ(...)) when shown.

• Consumer-facing compatibility note

  • Added an unreleased CHANGELOG note that .turbo-progress-bar now encodes progress via transform, so customizations that animate/read width should migrate accordingly.

refresh() {
  requestAnimationFrame(() => {
    const scale = (10 + this.value * 90) / 100
    this.progressElement.style.transform = `scaleX(${scale}) translateZ(0)`
  })
}