Add multi-consumer pause-reason API to SDK root

[ktsaou]

Summary

The SDK root's paused attribute was being written directly by each
caller — a UI hover toggle, an open modal, a long-running export.
Whichever caller flipped last won, and the "last flipped" was often
the wrong one: e.g., when the cursor moved from a topology canvas
onto an open actor modal, the canvas hover-end fired and wrote
paused: false, releasing the modal's still-active pause intent.
Updates resumed underneath the modal, tables and the mini-topology
jittered on every refresh tick.

The fix is multi-consumer pause-reason aggregation INSIDE the SDK,
exposed on every node via addPauseReason / removePauseReason.
Reasons compose: hover-end removes its own id and leaves the modal's
id in place, the aggregate stays true while the modal is open.

API

• node.addPauseReason(reasonId) — registers an opaque reason id.
• node.removePauseReason(reasonId) — removes it.
• paused is auto-recomputed as
  blurReason || pauseReasons.size > 0 whenever a reason is added/
  removed OR when the blur sources (blurred,
  autofetchOnWindowBlur) change.
• Idempotent on the same id, no-op for empty/missing ids, fully
  cleared on destroy().

Downstream

@netdata/cloud-frontend's usePauseSDK hook moves from its own
module-level Set to this API in netdata/cloud-frontend#5520. After
that PR lands and the SDK ships a version with this change, the
cloud-frontend hook becomes a thin wrapper that just pushes/pulls
its useId() through the SDK API.

Tests

8 new tests in src/sdk/makeNode.test.js cover the aggregation,
blur combination, idempotency, empty-id handling, and the original
clobber-bug regression (remove-one keeps aggregate true while
another remains).

Test plan

• yarn test src/sdk/makeNode.test.js passes
• No behavioral change for callers that haven't migrated —
  they continue to write paused directly via
  updateAttribute (the new API only adds reasons to a private
  Set; direct attribute writes still work the same way)
• Once cloud-frontend#5520 lands and consumes this, verify in
  the browser: open the actor modal on a topology view, move the
  cursor onto the modal, confirm updates STAY paused