ui: plan canvas ink extent + the keyboard-first round

Plan canvas (owner report: arrows clipped at the viewBox edge in dist —
measured 14px/20px cuts in the owner's real plans): blockLayout now
unions node boxes with every edge path's control-point hull (a bezier
never leaves it; the layout calls edgePathD with the renderer's exact
inputs). Dagre's node extent stays the floor, so ordinary plans render
byte-identical; the viewBox origin shifts only when ink bulges past
zero. Verified on both real chats; pins use the owner's real
linear-algebra tree (golden-real-data).

Keyboard-first (owner's law: Enter always submits, arrows always cycle;
text fields the only exception):
- Enter guard split (enter-submits): a focused BUTTON keeps native
  Enter only when keyboard-walked (:focus-visible) or inside an open
  overlay (menus/dialogs); pointer-clicked transcript buttons submit,
  preventDefault suppressing the re-activation underneath.
- Focus hygiene (focus-hygiene): pointer clicks on transient controls
  (agent/user thinking toggles, copy, mermaid expand, plan chip) blur
  to body — the state clicking empty space produces.
- Session search owns the walk: Up/Down wrap-cycle the filtered rows,
  Enter opens the walked row or the first match (quick-open), then
  blurs and keeps the query. The Alt-arrow session cycle is RETIRED.
- Registry: model-menu walk/pick entries removed (obvious in-field
  navigation, not settings material); question-dock keys stay
  displayOnly per owner ruling.

Gates: 1489/1489, typecheck, lint. Verified live end-to-end.

components/md-blocks.tsx

@@ -39,6 +39,7 @@ import {
 import type { ExtraProps } from "streamdown";
 import { MERMAID_FONT_SIZE, renderMermaid } from "@/components/mermaid-render";
 import { MermaidError } from "@/components/mermaid-error";
+import { blurAfterPointerClick } from "@/components/focus-hygiene";
 import { chartSize, useSessionMermaid } from "@/components/session-overlay";
 import { Hint } from "@/components/ui/hint";
 
@@ -101,7 +102,8 @@ export function CopyCorner({
 				aria-label={label}
 				data-testid="md-copy"
 				className="flex size-6 items-center justify-center rounded-md text-muted-foreground/70 opacity-0 transition-opacity hover:bg-accent hover:text-foreground focus-visible:opacity-100 group-hover/md:opacity-100"
-				onClick={() => {
+				onClick={(e) => {
+					blurAfterPointerClick(e);
 					void copyText(getText()).then((ok) => {
 						if (!ok) return;
 						setCopied(true);
@@ -210,7 +212,10 @@ function MermaidInline({ chart }: { chart: string }) {
 						aria-label="Expand diagram"
 						data-testid="md-mermaid-expand"
 												className="flex size-6 items-center justify-center rounded-md text-muted-foreground/70 opacity-0 transition-opacity hover:bg-accent hover:text-foreground group-hover/md:opacity-100"
-						onClick={() => overlay.openMermaid(chart)}
+						onClick={(e) => {
+							blurAfterPointerClick(e);
+							overlay.openMermaid(chart);
+						}}
 					>
 						<MaximizeIcon className="size-3.5" />
 					</button>