Fix transparent background flash during sidebar toggle (#2378)

* Fix transparent background flash during sidebar toggle

Move terminal background rendering from the Metal GPU pass to a
CALayer (backgroundView). The GPU bg_color pass is disabled via a
new Ghostty config flag (macos-background-from-layer). The CALayer
resizes instantly with its parent NSView, eliminating the 3-5 frame
gap where the desktop was visible through the transparent window
during sidebar toggles and layout transitions.

Also simplifies the titlebar and sidebar opacity formulas since
there is now a single background layer instead of two stacked
semi-transparent layers.

* Document macos-background-from-layer fork change

* Pin GhosttyKit checksum for macos-background-from-layer

* Address review feedback: fix fallback config path, inline identity wrapper

- Inject macos-background-from-layer in the fallback config path too,
  preventing alpha double-stacking when user config is invalid
- Inline panelBackgroundFillColor (now an identity function) at its
  two call sites and remove the wrapper

* Address adversarial review: skip fullscreen bg draw call explicitly

The bg_color uniform alpha is still zeroed for cell compositing (so
transparent cells pass through to the CALayer), but the fullscreen
background fill draw step is now explicitly skipped instead of relying
on alpha=0 as a no-op.

* Pin GhosttyKit checksum for bg draw-call skip

---------

Co-authored-by: Lawrence Chen <lawrencecchen@users.noreply.github.com>

scripts/ghosttykit-checksums.txt

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