core/qmlscreen: expose the screen refresh rate

[khughitt]

ShellScreen exposes a screen's geometry and density characteristics but not
its refresh rate, so QML has no way to find out how often a screen refreshes.
Shells that pace animations, budget work per frame, or reason about frame
timing currently have to get it from the compositor over an out-of-band
channel, if they can get it at all.

QScreen already tracks the value and emits a change signal for it, and it is
populated on Wayland. This forwards both, in the same shape as the surrounding
properties: a nullptr guard returning 0.0 for the dangling-screen case, and
a signal-to-signal connection alongside the existing ones in the constructor.

The doc comment notes that this is the rate reported for the current mode
rather than a measurement of when frames are actually presented, since the two
diverge under variable refresh and the distinction matters for the timing use
case that motivates the property.

Testing

• just fmt produces no changes; clang-format -Werror --dry-run passes on both files.
• just lint-changed equivalent (clang-tidy with tidyfox) reports no warnings in
  either file. Verified the lint was actually running on them by temporarily
  dropping a this-> in the new getter and confirming tidyfox-explicit-thisptr
  caught it.
• just test passes 9/9.
• Checked at runtime under niri: ShellScreen.refreshRate reads 59.999 for a
  DP-1 output whose mode niri reports as 59.999 Hz. It read undefined before.

QScreen::refreshRate and refreshRateChanged are unguarded and long predate
Qt 6, so this needs no version gating against the 6.6.0 minimum.

Contributions

• Claude Opus 5 + GPT 5.6-sol helped with the initial investigation, authoring the patch,
  testing / validation, and write the commit message + PR draft description
• The lazy human (me) reviewed the code line-by-line, checked to make sure we are abiding by the
  contributers guidelines and added this section to provide some clarity on who did what, and
  handled the actual PR submission.