fix(joint-react): prevent stale layout:update entries from resurrecting removed cells (dev)

[samuelgja]

Description

Dev-branch port of the externally-owned-graph undo fix (see the companion PR against
master: #3494). The reworked projection on dev (immutable snapshot
container with stageWrite/stageRemove pending buckets) carries the identical defect —
verified by running the same failing test against dev before the fix.

Same three invariants, adapted to the staged container:

• add/change entries are gated on graph membership; a stale entry repairs a lingering
  record via stageRemove (only when currentRecord(id) exists, so the incremental delta
  is not polluted with removals of never-present ids).
• remove entries only apply when the cell has actually left the graph (paper view-unmount
  notifications can name still-mounted cells, e.g. viewport culling).
• The orphaned-link sweep runs once per batch over the committed snapshot plus both pending
  buckets, keyed on the batch's removed-element set, and skips links the graph still holds.

Plus the same use-create-portal-paper.tsx recheck of pendingLinks after element-portal
commits, and the same test files.

Note for the future master → dev sync: this PR pre-resolves the merge for
graph-projection.ts — on conflict, take the dev side (it already contains the fix, and
the shared external-graph-undo.test.tsx verifies the resolution). No changeset here on
purpose: the master PR carries the release entry, and a second one would duplicate the
changelog when dev merges down.

Motivation and Context

Same customer report as the master PR: delete an element with a link, undo, and the
restored element renders empty with its link permanently hidden. master is the patch line
for the affected 4.3.5 release; this keeps dev's state rework from shipping the same bug.

Screenshots (if appropriate):

[Copilot]

🟡 Changes recommended

The new remove handling can emit incremental “removed” ids for records that never existed in the container (data-less layout:update removals), which can break consumers relying on accurate deltas.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

Ports the external-graph undo fix to dev’s staged snapshot container so stale layout:update entries can’t resurrect removed records, and so link records don’t remain stranded/hidden after delete+undo in externally-owned graphs.

Changes:

• Harden graphProjection against stale add/change/remove entries by gating writes/removals on current graph membership and sweeping orphaned link records once per removal batch.
• Add/adjust tests to reproduce the stale-record + missed-link-removal scenarios and assert container correctness.
• Recheck pendingLinks after element-portal React commits so links parked hidden by insertView don’t remain hidden indefinitely.

File summaries

File	Description
packages/joint-react/src/store/graph-projection.ts	Adds graph-membership gating for staged writes/removals and a post-batch orphan-link sweep to prevent stale records from resurfacing.
packages/joint-react/src/store/tests/graph-projection-edge-cases.test.ts	Updates edge-case test to simulate a missed link removal and verify the new orphan-link sweep behavior.
packages/joint-react/src/hooks/use-create-portal-paper.tsx	Adds a useEffect to re-run checkPendingLinks() after portal content commits.
packages/joint-react/src/components/graph/tests/external-graph-undo.test.tsx	New integration-style tests covering delete+undo and stale layout:update races for externally-owned graphs.

Review details

• Files reviewed: 4/4 changed files
• Comments generated: 1
• Review effort level: Lite

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