spec(jump-overlay): "open labelled video in a new tab" mode — tab-open mechanism unverified across browsers (no implementation)

[Kotmin]

Goal

Add a jump-overlay mode that opens the labelled video in a new tab within the
same window (a "card"), leaving the current video playing in place.

Distinct from the queue overlay (Ctrl+A p), which enqueues for watch-next in
the same tab. This is for parallel browsing.

Proposed shape (not agreed — needs the mechanism question resolved first)

• Reuse collectJumpTargets / the existing createJumpOverlay; no new overlay
  component.
• New chord (candidate: Ctrl+A t) opens the jump overlay filtered to video
  links only: a[href*="/watch"], a[href*="/shorts/"].
• On exact label match, open that href in a new tab instead of calling
  activateTarget (which navigates in place).
• New config key in src/core/shortcuts.config.json; new row in
  docs/keyboard-quickstart.md in the same commit.

Open question — the tab-open mechanism (blocks implementation)

Requirement: same window, new background/foreground tab, works on all
supported editions (Chrome, Edge, Firefox) and ideally degrades sanely on Arc.

Candidates, none verified:

1. window.open(href, '_blank') from the content script.
   • No tabs permission, no background messaging.
   • Unverified: does every target browser honour "same window, new tab" vs
     spawning a popup window? Behaviour under popup blockers when not in a
     direct user-gesture call stack? Firefox vs Chromium differences?
2. Synthetic ctrl/meta-click on the anchor — rejected: isTrusted is
   false, Chromium will not perform the native new-tab navigation.
3. chrome.tabs.create({ url, windowId: current }) via the background service
   worker + a content->background message.
   • Guarantees same-window placement and gives active/index control.
   • Cost: tabs permission (store-review + privacy note) and message
     plumbing. Only justified if option 1 proves unreliable.

Deliverable for this issue

Decide the mechanism with real evidence:

• Test window.open(href, '_blank') behaviour in Chrome, Edge, Firefox (and
  note Arc) from within a content script, both inside and outside a keydown
  gesture call stack.
• Confirm whether it lands as a same-window tab in every case.
• If not reliable, scope the chrome.tabs.create route and its permission
  impact against issues spec(chrome-edge): Chrome/Edge Web Store parity, domain-separation guard, and PRD Milestone 9-10 scoping (no implementation) #6 / spec(opera-safari): Opera Add-ons and Safari Web Extension distribution scoping (no implementation) #12 (store distribution scoping).

No implementation until the mechanism is chosen.