Bite-sized task breakdown for the spec at
docs/design/docs/docs-peer-jump.md: five tasks covering the new
EditorAPI.scrollToPosition method, UserPresence generalisation +
Sheets migration, DocsView jump handle, DocsLayout wiring, and
manual verification + archive.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

Domain-agnostic helper that smooth-scrolls the viewport so a given
DocPosition appears roughly one-third from the top. Reuses the
existing resolvePositionPixel helper that backs peer cursor rendering
and Cmd+F result jumps. Silent no-op before the first paint or when
the blockId is stale.

Caches canvasHeight and logicalCanvasWidth in initialize() closure
scope so the new method can read them outside the render pipeline.

UserPresence no longer knows about Sheets-specific activeCell. The
component now emits a domain-agnostic onSelectPeer(clientID) event
and asks the host for a tooltip hint via getJumpHint(clientID). This
is the precondition for wiring docs peer-jump in a follow-up commit.

Sheets call sites in document-detail.tsx and shared-document.tsx are
migrated to the new API. Each picks up the click, looks up the peer
in Yorkie presence, and runs the same tab-resolution + peer-jump-
target logic as before. Pure refactor — no behavioural change.

Pull the duplicated hint/canJump computation out of renderAvatar and
the +N dropdown into a tiny resolveHint helper so the pattern lives
in one place. Replace onSelectPeer!() non-null assertions with an
explicit early return that lets TypeScript narrow the prop. Pure
refactor — no behavioural change.

DocsView gains a jumpToPeer(clientID) callback that resolves the
peer's activeCursorPos from Yorkie presence and asks the editor to
scrollToPosition there. Also resets the 4-second peer-label timer so
the username briefly re-appears at the landing position.

The handle is published via a new optional onJumpHandleReady prop;
no consumer wires it up in this commit.

The prop comment claimed 'peer-jump, scroll' but the JumpHandle
exposes only jumpToPeer. Drop the misleading 'scroll' clause.

DocsLayout now holds a JumpHandle from DocsView and forwards peer
avatar clicks through it. getJumpHint reads the peer's username from
DocsPresence and gates the click affordance on activeCursorPos being
present. Only the viewport scrolls — the local caret is unaffected.

Manual verification deferred to PR review (recorded in the lessons
file). All five plan tasks shipped behind pnpm verify:fast green
across each commit.