G4: bloom, vignette and grain in the post chain

Three bundled Shadertoy passes beside Crt, each `Bloom|Vignette|Grain
0..3` (off by default, nothing drawn while off), in shaders/post/:

- Bloom: what is brighter than the page (a channel's linear peak over
  0.75/0.7/0.6 and over the page's own + 0.15, so a light page does not
  bloom) glows, 2/3.5/5% (§8.2's budget). The glow is a dual Kawase blur
  in RGBA16F textures at half size and down, 3/4/5 steps by level
  (shaders/bloom-down, bloom-up; Post.bloomChain), composited in linear
  light and dithered.
- Vignette: round, from halfway to a corner, 10/18/28% at the far corners,
  in linear light, dithered.
- Grain: a still, triangular-spread grain of 1.5/3/5 of 255 on
  page-coloured pixels only, a logical pixel big; still, so it asks for no
  frames.

Focus rule: the shell passes every bundled pass the rectangles of the tags
with their grips, the column and workspace tags, the notices and the
cursor (pardesSpare, at most 32, neighbours on one row merged), and a
bundled pass leaves them as they are; and a per-theme ceiling (pardesCap,
Post.capsOf) keeps the page's text and the selection's text at min(their
contrast, 4.5) under each pass at its strongest. User files get no
ceiling and no spared rectangles.

Fix found on the way: SDL GPU binds at most 4 KiB of a uniform block, so
every Shadertoy colour after iPalette (iBackgroundColor, iForegroundColor,
the cursor and selection colours, iPalette[>=4]) read zero. The prefix's
block is cut in three (Globals, Palette, Colors) with ghostty's names and
order, each pushed as a slice of the one struct; pardes's own block moves
to binding 3.

Tests: the caps hold over every native theme; each uniform slice fits
4 KiB; the prefix's members in ghostty's order across its blocks.
Gates: tty/gui unit-test and core-test pass; snap fails only
nested-optout; GUI goldens 01-08 byte-identical (09-18 wait on
wkzlsqvy, as in somtrmsz); web builds. Feel review material:
.scratch/render/{bloom,vignette,grain}/ (levels on forge, dusk, acme,
crops, a levels mp4).

1 files changed, 42 insertions, 0 deletions

diff --git a/src/gui/gui.zig b/src/gui/gui.zig
index fdcd7b2a..bde9aecd 100644
--- a/src/gui/gui.zig
+++ b/src/gui/gui.zig
@@ -5732,8 +5732,10 @@ fn renderFrame(
         if (core) |p| {
             var boxes: [max_cursors]CursorBox = undefined;
             const cursors = if (cursors_shown) cursorBoxes(g, surface, layout, win_h, &boxes) else 0;
+            var spare: [Post.max_spare][4]f32 = undefined;
             g.post.describe(.{
                 .cursor = if (cursors > 0) boxes[0] else null,
+                .spare = spare[0..postSpare(surface, layout, &spare)],
                 .background = chrome.page orelse bg_default,
                 .foreground = chrome.fg orelse fg_default,
                 .selection_foreground = p.theme().sel_fg,
@@ -8594,6 +8596,46 @@ test "decor is read from the regions, each pane's in its own group" {
     try std.testing.expectEqual(@as(u32, 0), groups.items[groups.ink].decor_count);
 }
 
+/// What a bundled post pass leaves alone (Post.Frame.spare), in window
+/// pixels: every tag with its grip, the column and workspace tags, and the
+/// notices, the chrome the focus indicators live in. A grip, the gap after
+/// it and the tag beside it are one rectangle; past the list's end the last
+/// grows to cover the rest.
+fn postSpare(surface: *const pardes.Surface, layout: CellLayout, out: *[Post.max_spare][4]f32) usize {
+    var n: usize = 0;
+    for (surface.regionList()) |region| {
+        switch (region.kind) {
+            .grip, .tag, .notice, .column_grip, .column_tag, .workspace_tag => {},
+            else => continue,
+        }
+        if (region.rect.w == 0 or region.rect.h == 0) continue;
+        const rect: [4]f32 = .{
+            @as(f32, @floatFromInt(region.rect.x)) * layout.w,
+            @as(f32, @floatFromInt(region.rect.y)) * layout.h,
+            @as(f32, @floatFromInt(region.rect.x + region.rect.w)) * layout.w,
+            @as(f32, @floatFromInt(region.rect.y + region.rect.h)) * layout.h,
+        };
+        const joined = for (out[0..n]) |*other| {
+            // The same rows, a gap cell or two apart (a grip, the gap after
+            // it, its tag): one band.
+            const gap = 2 * layout.w + 0.5;
+            if (other[1] == rect[1] and other[3] == rect[3] and rect[0] <= other[2] + gap and other[0] <= rect[2] + gap) {
+                other.* = .{ @min(other[0], rect[0]), rect[1], @max(other[2], rect[2]), rect[3] };
+                break true;
+            }
+        } else false;
+        if (joined) continue;
+        if (n == out.len) {
+            const last = &out[n - 1];
+            last.* = .{ @min(last[0], rect[0]), @min(last[1], rect[1]), @max(last[2], rect[2]), @max(last[3], rect[3]) };
+            continue;
+        }
+        out[n] = rect;
+        n += 1;
+    }
+    return n;
+}
+
 const CursorBox = Post.Frame.Cursor;
 const max_cursors = 1 + pardes.MAX_PANES + pardes.MAX_TAG_LAYERS;