Fix comparisons of unchanged linked trait values

[MbappeWU]

Fixes #756.

The post-propagation consistency checks compare a trait value with change.new even when they are still the same object. For values such as pandas DataFrames, inequality produces a non-scalar result and its truth conversion raises, interrupting normal link updates and later observers.

Add the protected _should_update comparison hook suggested in the issue. Its default accepts object identity and retains inequality for distinct values. Both consistency checks use the hook, so subclasses can customize comparison without replacing the propagation methods. Existing scalar equality and broken-link detection remain intact.

The regressions exercise both directions, observer continuation, equal-but-distinct replacements, custom comparison through actual propagation, and recovery of the updating flag. The guide documents that this is a consistency check after writing the other trait, rather than a condition that suppresses the write.

The upstream Tests run passes all 26 tasks, including Linux/Windows/macOS, PyPy and free-threaded Python, minimum and prerelease dependencies, typing/lint, docs, source-archive installation, the dry-run release check, and all five downstream projects.

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[codecov]

Codecov Report

❌ Patch coverage is 98.43750% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 93.27%. Comparing base (c4f1247) to head (34fc553).

Files with missing lines	Patch %	Lines
tests/test_traitlets.py	98.27%	0 Missing and 1 partial ⚠️

Components	Coverage Δ
traitlets	85.70% <100.00%> (+0.06%)	⬆️
tests	99.06% <98.27%> (-0.01%)	⬇️

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