Fix stale Gaussian splat handles after set_visible_worlds()

[aneangel]

Description

set_visible_worlds() left stale, still-visible Gaussian splat handles
behind in ViewerViser (and any other backend that tracks handles by
name).

Gaussian shape names were generated using len(_gaussian_instances) as
a sequential counter. That counter resets to zero every time
set_visible_worlds() rebuilds the list, so world N's shape would steal
world 0's old backend name. World N's original name became an orphan
in _scene_handles — its handle was never visited again, so it was
never hidden or removed. The result was a frozen duplicate rendering at
the shape's last known position alongside the live copy.

Two changes fix this:

1. Stable names — use the global shape index s (its position in
   the model's fixed shape array) as the name suffix instead of
   len(_gaussian_instances). Each shape's name is now invariant across
   set_visible_worlds() calls.
2. Cleanup — after repopulating _gaussian_instances, hide handles
   for any Gaussian name that is no longer in the active set.

Closes #4020

Checklist

• New or existing tests cover these changes
• The documentation is up to date with these changes
• For user-facing changes, a fragment has been added by following the
  changelog fragment instructions

Test plan

uv run --extra dev newton/tests/test_viewer_log_shapes.py
uv run --extra dev newton/tests/test_viewer_visible_worlds.py
uv run --extra dev newton/tests/test_viewer_geometry_batching.py

All pass with no failures.

Bug fix

Steps to reproduce (without this PR):

1. Build a model with two worlds, each containing one Gaussian shape.
2. Render one frame with both worlds visible.
3. Call set_visible_worlds([1]) and render another frame.
4. World 1 now renders twice: once live at its current position, and
   once as a frozen duplicate stuck at its prior position.

Minimal reproduction: see the script in issue #4020.

Summary by CodeRabbit

• Bug Fixes

  • Fixed stale Gaussian visualization handles remaining visible after changing the set of visible worlds.
  • Ensured Gaussian shape identifiers remain stable across visibility updates.
  • Improved cleanup when worlds are hidden or removed from the current view.

• Documentation

  • Added a changelog entry describing the visibility-handling fix.

[coderabbitai]

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ℹ️ Recent review info

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📥 Commits

Reviewing files that changed from the base of the PR and between bdf01ba and fb9ac5f.

📒 Files selected for processing (2)

• changelog/4020.fixed.md
• newton/_src/viewer/viewer.py

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📝 Walkthrough

Walkthrough

set_visible_worlds() now preserves Gaussian instance names across repopulation and hides handles for Gaussian instances that are no longer visible. A changelog entry documents the fix.

Changes

Gaussian visibility fix

Layer / File(s)	Summary
Stable naming and stale-handle cleanup newton/_src/viewer/viewer.py, changelog/4020.fixed.md	_populate_shapes derives Gaussian names from stable shape indices. set_visible_worlds snapshots previous instances and hides handles absent from the newly populated set. The changelog records the fix.

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: 🔵 Low · up to fb9ac

The change fixes stale Gaussian renders by preserving stable shape identities and hiding handles that are no longer active, but an interrupted visibility rebuild could still leave obsolete handles visible and unreachable for cleanup; the PR is mergeable with explicit owner awareness of this bounded failure-path risk.

Suggested reviewers: nvtw

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 50.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 2 functions across 1 files. (1 skipped: 1…	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	The changes address issue #4020 by assigning stable Gaussian shape names and hiding handles for Gaussian instances that are no longer active after visibility changes.
Out of Scope Changes check	✅ Passed	The code and changelog changes directly support the stale Gaussian handle fix described in issue #4020. No unrelated changes are present.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly identifies the main change: fixing stale Gaussian splat handles after set_visible_worlds().

Full details: Docstring Coverage

Explanation

Docstring coverage is 50.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 2 functions across 1 files. (1 skipped: 1 unsupported.)

• Fix all pre-merge checks with AI

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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