The GUI draws in tiers from the regions; pane chrome moves with its pane

Stage 8 of docs/render-pipeline.md. The frame is drawn in groups: tier 0,
one per track in paint order (tiers 2 and 3), tier 4 (notices, then guides
and the debug box), tier 5 (bar cursors), each cells, images, decor. Decor
(rules, rails, thumbs, grip marks, spines, the workspace and column rules,
the bottom band, notice rules, bar cursors) is whole-pixel rects through
the new decor.frag over ui.vert, so it carries its track's transition and
clip. A closing pane's chrome comes from Surface.previous_regions.

Deleted: taglineBaseRgb, topbarPaneBorderHeight, bottomTaglinePresent,
frameChromeBg, cellBackgroundIs and the rail inference, paneGripCell's
scan, transient_on, PaintPlan. One cover map (coverFrame) is marked from
the layers and regions once a frame. New regions column, guide and debug;
Surface.chrome is the palette, carried in wire v8 (not shipped yet), so an
attached GUI draws the same chrome (test). A per-instance clip replaces the
vertical transition's scissor.

Goldens: 01-12 byte-identical. 13-17 differ only past the grid: the image
pass left its scissor at the grid's size, cutting rule ends, rail feet and
the bottom band in the leftover pixels whenever a picture was on screen.
16-debug now shows the debug box, which a context-row layer had hidden.
18-mid-transition is new: virtual clock (PARDES_TEST_CLOCK in the GUI),
PanelSlide Newcol with the picture, frame 6 of 12.

Also: the GUI sleeps when idle instead of polling every 16 ms, and a
minimized or occluded window sleeps through animation. Tracy 'gui frame
build' at 200x60: 992/1015 us median before, 989/987 us after.

Shared files touched: pardes.zig (one export), detached/client.zig,
detached/server.zig, detached/wire.zig (all additive), gui.zig. Not touched:
Messages.zig, mouse.zig, tagline.zig, colors.zig, tty.zig, host_io.zig,
dump.zig, exec.zig, panes.zig.

1 files changed, 6 insertions, 0 deletions

diff --git a/shaders/ui.vert.glsl b/shaders/ui.vert.glsl
index d23b609c..c1779add 100644
--- a/shaders/ui.vert.glsl
+++ b/shaders/ui.vert.glsl
@@ -19,12 +19,17 @@ layout(location = 5) in vec4 a_present_rect;
 // the id is what is left below them; integer transport preserves the exact
 // cross-backend hash.
 layout(location = 6) in uvec4 a_effect;
+// A clip rectangle in target pixels (x0, y0, x1, y1): the fragment shader
+// drops what lies outside it. x1 < x0 is none. A vertical transition clips
+// its pane's instances to the box the pane is growing into.
+layout(location = 7) in vec4 a_clip;
 
 layout(location = 0) out vec2 v_uv;
 layout(location = 1) out vec3 v_fg;
 layout(location = 2) out vec3 v_bg;
 layout(location = 3) out vec2 v_cell_uv;
 layout(location = 4) flat out uvec4 v_effect;
+layout(location = 5) flat out vec4 v_clip;
 
 void main() {
     vec2 pos;
@@ -70,5 +75,6 @@ void main() {
     v_fg = a_fg;
     v_bg = a_bg;
     v_effect = a_effect;
+    v_clip = a_clip;
     gl_Position = vec4(pos, 0.0, 1.0);
 }