Let WindowOpacity through on macOS

The builtin was gated on the SDL platform, so the macOS shell never registered
it and the setting had nowhere to land. macOS already had transparency, but
only the binary kind a theme decides — a theme with no background of its own
drops the ground and the blur shows through. WindowOpacity is the graded,
theme-independent version, and the window had no way to hear about it.

pardes_window_opacity reports the percentage and acknowledges the request in
one read, beside pardes_theme_bg: an SDL surface can arrive without an alpha
channel and has to be able to refuse, while an AppKit window always composites
per pixel, so there is nothing here to refuse and no rollback to perform.

The view then paints the rule shaders/ui.frag.glsl states for the SDL shell:
backgrounds — ground, cell and band fills, and the chrome rules over them —
take the alpha, glyph ink never does, and the block cursor is exempt because
it is foreground chrome that happens to be carried in a cell background. That
exemption is why the cursor joins the colour in the background run key; it can
no longer share a fill with the cells beside it.

Two things the harness was missing fall out of testing it: it never adopted
the theme background or the opacity, so a Theme or WindowOpacity in a script
moved the core and never reached a pixel. draw-opacity records both ends of
the alpha range, which is what makes the two-sided contract assertable —
60% reads 153..255, and 0% reads 0..255 with the ink still standing.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

2 files changed, 63 insertions, 0 deletions

diff --git a/test/macos-snapshots/opacity.golden b/test/macos-snapshots/opacity.golden
new file mode 100644
index 00000000..b28c6e0c
--- /dev/null
+++ b/test/macos-snapshots/opacity.golden
@@ -0,0 +1,29 @@
+== draw-opacity solid 680x396 alpha 255..255
+== snap translucent grid=80x24 cursor=4,3
+|Newcol Joincol Find Grep Help Changelog Tutor Dump NextColor Debug Kill
+|   New Tty Find Grep Joincol
+|$  /private/tmp/pardes-macos-e2e/opacity/cwd Tty Save Del Mode Filter Collapse
+|  $
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+== draw-opacity translucent 680x396 alpha 153..255
+== draw-opacity clear 680x396 alpha 0..255
+== draw-opacity restored 680x396 alpha 255..255
diff --git a/test/macos-snapshots/opacity.snap b/test/macos-snapshots/opacity.snap
new file mode 100644
index 00000000..0f73cb17
--- /dev/null
+++ b/test/macos-snapshots/opacity.snap
@@ -0,0 +1,34 @@
+# WindowOpacity, end to end: the builtin the core only offers to a shell that
+# declared the capability (src/builtins.zig), through pardes_window_opacity,
+# to the alpha CoreGraphics actually left in the framebuffer.
+#
+# The contract is two-sided and shaders/ui.frag.glsl states it for the SDL
+# shell: BACKGROUNDS take the percentage, glyph ink never does. `draw-opacity`
+# records both ends of the range so a regression that made the whole window
+# translucent — text included — reads as a changed maximum, not as a pass.
+start 24 80
+wait 8000 Newcol Joincol
+wait 8000 $
+stable 700 20000
+
+# Opaque is the default, and an opaque view caches without an alpha channel
+# at all.
+draw-opacity solid
+
+# Backgrounds drop to 60%; the prompt and the chrome text stay at 255.
+command WindowOpacity 60
+stable 700 20000
+snap translucent
+draw-opacity translucent
+
+# 0 is the extreme the block cursor exists to survive: every background is
+# gone and the ink is still there.
+command WindowOpacity 0
+stable 700 20000
+draw-opacity clear
+
+# ...and back, which must restore an opaque view rather than leave a window
+# that is merely painting 100% alpha into a transparent surface.
+command WindowOpacity 100
+stable 700 20000
+draw-opacity restored