Context

src/snapshot/diff.zig has a working diffSnapshots() function but no HTTP endpoint exposes it.

Implementation

1. Add /diff/snapshot?tab_id= route in router.zig
2. Store previous snapshot per tab in Bridge (or use existing snapshot cache)
3. Return delta: added/removed/changed nodes
4. agent-browser equivalent: diff snapshot

Acceptance Criteria

• Navigate, snapshot, navigate again, call /diff/snapshot → returns meaningful delta
• Supports optional previous snapshot param

Co-authored-by: blackfloofie 265516171+blackfloofie@users.noreply.github.com