opentui(v6): clamp negative draw coords at the node:ffi seam (diff crash fix)

Expanding a tall <diff showLineNumbers> pinned to the scrollbox bottom froze
the TUI with ERR_INVALID_ARG_VALUE looping out of CliRenderer.loop every
frame. Root cause: @opentui/core 0.4.0 marshals OptimizedBuffer
fillRect/drawText/setCell* coordinates as u32 in the FFI table while
LineNumberRenderable.renderSelf passes raw screen coordinates — NEGATIVE when
the diff is partially scrolled above the viewport. Bun's FFI silently wraps
negatives (native side bounds-checks them into a no-op); Node's experimental
node:ffi rejects them. bufferDrawBox already uses i32, which is why ordinary
boxes/text scroll fine and only the diff line-background path crashed.

Fix at the seam we own: boundary/ffiSafe.ts patches OptimizedBuffer to clip
fillRect to the non-negative quadrant and skip negative-origin
drawText/setCell*/drawChar before the FFI call (Bun parity). Installed from
boundary/renderer.ts (live) and test/lib/render.ts (headless). TODO(upstream):
widen those FFI params to i32 so this shim can be deleted.

ui-opentui/src/test/lib/render.ts

@@ -23,6 +23,13 @@ import { testRender, useRenderer } from '@opentui/solid'
 import type { JSX } from '@opentui/solid'
 import { createMemo } from 'solid-js'
 
+import { installFfiCoordSafety } from '../../boundary/ffiSafe.ts'
+
+// Headless renders go through the same node:ffi seam as the live TUI — install
+// the negative-coordinate shim here too (the live path installs it in
+// boundary/renderer.ts, which tests don't import).
+installFfiCoordSafety()
+
 /** Wrap a node in a KeymapProvider whose keymap is bound to the test renderer. */
 function withKeymap(node: () => JSX.Element): () => JSX.Element {
   return () => {