feat(vk): draft the VK community cover

Adds VK's own cover template (1920x768) and a self-contained generator that
renders an Эрудит board in perspective with ГРАМОТНОСТЬ / СОПЕРНИК /
ЭРУДИЦИЯ / УМ already played, in six art-direction variants for review.

The position is checked, not asserted: assertLegal re-reads the finished grid
and fails the build unless the centre is covered, every declared word is on the
board, and no maximal run of two or more letters is a word nobody played. All
four words are in the Эрудит dictionary and tile values come from
scrabble-solver rules.Erudit().

The camera has pitch only, so the perspective warp reduces to an exact
per-scanline remap of a flat texture; tile thickness is filled as 3D quads
between the board plane and the tile plane. skia-canvas is borrowed from the
render sidecar rather than installed a second time for a one-off asset build.

Composition respects the two VK keep-outs measured off the template: the top
130 px some clients crop, and the mobile avatar circle over the bottom-left —
which is why the default word arrangement keeps that corner empty.

assets/vk/cover/README.md

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+# VK community cover
+
+The cover art for the VK community (`1920×768`). The community avatar — the «Э» tile — is
+uploaded separately; this is only the backdrop behind it.
+
+`build-cover.mjs` is the whole thing: a small software renderer that draws an Эрудит board in
+perspective with the opening `ГРАМОТНОСТЬ / СОПЕРНИК / ЭРУДИЦИЯ / УМ` already played, and
+emits one PNG per art-direction variant plus a contact sheet.
+
+```sh
+cd assets/vk/cover
+node build-cover.mjs                # every variant -> preview/
+node build-cover.mjs night-spot     # one variant
+FAST=1  node build-cover.mjs        # 1x instead of 2x supersampling, for quick iteration
+ZONES=1 node build-cover.mjs        # overlay the VK keep-outs on the render
+LAYOUT=cross node build-cover.mjs   # the alternative word arrangement
+```
+
+It needs no install: skia-canvas comes from the render sidecar's `node_modules`, and the
+letters are set in `Spectral-Bold.ttf` from the icon brand folder.
+
+## The position
+
+`ГРАМОТНОСТЬ` opens through the centre square (its `Т` on H8) and the other three words hook
+onto a shared letter. `assertLegal` re-reads the finished grid and fails the build if the
+centre is uncovered, if a declared word is missing, or if any maximal run of two or more
+letters is a word nobody played — so the board on the cover is always a position the engine
+would accept. All four words are in the Эрудит dictionary; tile values come from
+`scrabble-solver` `rules.Erudit()`.
+
+The default `ladder` arrangement puts both long words horizontally. The `cross` arrangement
+hangs `ЭРУДИЦИЯ` down the left instead, which looks more symmetric but collides with the VK
+mobile avatar (below).
+
+## VK keep-outs
+
+Measured off VK's own template (`../VK_group_cover_template.fig`), in cover pixels:
+
+| Zone | Area | Why |
+|------|------|-----|
+| Top crop | `y < 130` | trimmed by some clients — nothing important there |
+| Mobile avatar | circle, centre `(283, 818)`, r `283` | the community picture sits over the bottom-left in the VK app |
+
+`ZONES=1` paints both so a composition can be checked against them.
+
+## Rendering technique
+
+The camera has pitch only — no roll, no yaw — so a board-space horizontal line always maps to
+a screen-space horizontal line. That makes the perspective warp an exact per-scanline remap of
+a flat texture rather than a general homography: the board surface and the tile faces are each
+drawn once, flat, at high resolution, then replayed band by band (`warpPlane`). Tile thickness
+is filled as real 3D quads between the two planes, so the tiles have visible edges.
+
+Two things that are easy to get wrong here and cost a hundredfold in render time:
+`ctx.filter = 'blur(...)'` applies per drawing call, so many blurred shapes must be collected
+into one offscreen layer and blurred once (`blurred`); and `drawImage` re-snapshots a live
+`Canvas` source on every call, so a texture sampled hundreds of times must be baked into an
+`Image` first (`freeze`).