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, 15 insertions, 1 deletions

diff --git a/docs/config.md b/docs/config.md
index 40d0fb24..b3cff3a4 100644
--- a/docs/config.md
+++ b/docs/config.md
@@ -122,7 +122,7 @@ such a shell there when it is the column's only one and nobody has typed
 into it (no scrollback, cursor still on the first prompt line), the boot's
 placeholder giving its rows to the document. Bare `BootShell` flips it, as
 bare `Placement` does: a setting that chooses among words (`on`/`off`
-switches, `Placement`, `BootShell`, `Crt`, `Lift`, `Motion`,
+switches, `Placement`, `BootShell`, `Crt`, `Bloom`, `Vignette`, `Grain`, `Lift`, `Motion`,
 `ShaderAnimation`) steps to its
 next value when written bare, as its word clicked in a tag does, and a value
 it does not take is refused with the values it takes, which `/commands` also
@@ -542,6 +542,20 @@ the chain empty the pass is bypassed. CRT works in linear light with restrained
 scanlines, mask, bloom and vignette, and no curvature, so clicks land where
 they are drawn.
 
+Three more bundled passes take the same levels, each off by default and
+each costing nothing while off:
+
+```text
+Bloom 2      the brightest ink glows a little (only what is brighter than
+             the page; a dual Kawase blur at half size and down)
+Vignette 2   the window's corners fall a little into shade
+Grain 2      the page's own ground takes a fine, still grain, like paper
+```
+
+None of them touches a tag, a grip, a notice or the cursor, and each is
+capped per theme so text and the selection keep their own contrast or 4.5
+(docs/effects.md).
+
 The focused pane can stand off the page (SDL GUI, off by default):
 
 ```text