fix: preserve RMSD reference after reconnect

[vvsrinath]

This PR preserves the RMSD reference when the dashboard reconnects.

Previously, _update_selection() always replaced reference_positions with the current positions of the newly connected Universe. This caused the RMSD reference to change after reconnecting.

The implementation adds reset_reference to _update_selection(). Reconnection uses reset_reference=False, preserving the existing reference, while changing the RMSD selection continues to reset the reference.

Added a regression test covering the reconnect scenario.

Focused test result: 5 passed, 68 deselected.

The remaining test-suite issue appears to be related to an environment dependency and is being investigated separately.

[read-the-docs-community]

Documentation build overview

📚 mdadash | 🛠️ Build #34184553 | 📁 Comparing a1d1b1f against latest (5780a37)

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1 file changed

±_modules/mdadash/backend/analyses/rmsd.html

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (5780a37) to head (a1d1b1f).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

Components	Coverage Δ
frontend	100.00% <ø> (ø)
backend	100.00% <100.00%> (ø)

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