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feat(cse): Python CSE Machine via Conductor (#4000)

* feat(cse): wire conductor CSE machine snapshots into frontend

Ports the language-agnostic CSE machine snapshot pipeline from the
liveness prototype into the main frontend, enabling Python (and any
future language) to display CSE machine visualizations via Conductor.

- Add CseMachineHostPlugin concrete subclass (reassignable receiveSnapshots)
- Add CseSnapshotAdapter to translate CseSnapshot → fake js-slang objects
- Register CseMachineHostPlugin in createConductor; return csePlugin
- Add cseSnapshots state field + updateCseSnapshots Redux action/reducer
- Add handleCseSnapshots saga fork in evalCodeConductorSaga; inject
  /__cse_config__ step-limit file so the evaluator respects the slider
- Add CseMachine.renderSnapshot(), getPairCreationMode(), lastSnapshot
- Add Layout.snapshotMode to skip Source-specific tree mutations
- Wire cseSnapshots prop + setEditorHighlightedLinesStep in SideContentCseMachine
- Add renderSnapshotAt, getChangepointSteps, envFingerprint methods
- Guard all CseMachine.redraw() calls to use renderSnapshotAt in snapshot mode

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* fix(lint): sort imports in CseMachine and SideContentCseMachine

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* fix(cse): pass csePlugin through conductor cache; port snapshot-mode animation fixes

- conductorEvaluatorCache: store and return csePlugin from PreparedConductor;
  was undefined before, causing 'Cannot set properties of undefined (receiveSnapshots)'
- CseMachineUtils.isEnvEqual: null-guard env1/env2 (snapshot adapter may produce
  closures whose defining env wasn't serialized)
- CseMachineAnimation: use __snapAnimType/__snapBody for snapshot-mode nodes;
  guard numOfArgs=0 for Python calls; use env identity for frameCreated detection;
  skip LookupAnimation for global-frame bindings; cast expression for ExpressionStatement
- CseMachineLayout: wrap CseAnimation.updat... (continued)

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