Story-2348: F toggles focus mode

Focus mode — the viewer taking the whole frame — was mouse-only to enter (the expand button on the strip) and keyboard-only to leave (Esc). That asymmetry is the bug. Counsel who reads a 200-page compendium with the arrow keys should never have to go find a button to get the screen out of the way. One key, both directions: F takes the frame, F gives it back.

The idea in one line

One map, one owner. The key is declared in the reading key map alongside the arrows, and acted on by the shell — because focus is the shell's state, and every lens deserves it.

Why the key goes in the map even though the map does not act on it

Story-2342 will ship a shortcuts overlay that lists the Reading Room's keys. It reads readingKeyIntent. A key that hides in its own private handler is a key counsel is never told about — so F joins the map as an intent, exactly like ArrowRight:

export type ReadingKeyIntent =
  | { kind: "scroll"; delta: number }
  | { kind: "viewport"; direction: 1 | -1 }
  | { kind: "step-page"; direction: 1 | -1 }
  | { kind: "jump-page"; edge: "first" | "last" }
  | { kind: "toggle-focus" };   // NEW — handled by the SHELL, on every lens

Why the SHELL acts on it, not the reading column

useReadingKeyboard is mounted by document-scroll-column — which only exists on the Original lens. Focus mode is not an Original-lens feature: counsel reading extracted text or the Insights dossier wants the full frame just as much. And focus is state the shell owns (it is the shell that goes fixed inset-0).

So the shell binds a second, tiny hook from the same module, and the column's hook explicitly steps over the intent. That skip is not decoration — it is what stops F from firing twice on the Original lens (once per listener), which would toggle focus on and straight back off.

// use-reading-keyboard.ts — inside useReadingKeyboard (the column, Original only)
const intent = readingKeyIntent(event);
if (!intent || intent.kind === "toggle-focus") return;   // the shell owns this one

// use-reading-keyboard.ts — the new hook (the shell, every lens)
export function useFocusKey(toggleFocus: () => void): void { ... }

// document-permalink-shell.tsx
useFocusKey(toggleFocus);

The guards — the same three, not a new set

Esc is untouched

Esc is still bound by the shell only while focus is on, and is still deliberately absent from the key map — focus mode owns it, and a second claim on it is how a key conflict starts. This Story adds a key; it does not renegotiate that one.

Evidence

Honest seams (verification doctrine)

Observability

Instrumentation-exempt per pramaan-docs ADR-124: a client-only keyboard handler inside an existing component tree. No new API route, handler, job, workflow, or user-visible server state. No new span.