Refactor/perf batch 3

[timcadman]

What's changed

• remove isDefined() and checkClass() and replace with serverside checks
• Class checking replaced by .checkClassConsistency() run on the class field already embedded in the function's normal result — no extra round-trip
• ValidityMessage handling removed
• Adapted to new list-based server returns where class is also returned (dimDS/lengthDS/numNaDS)
• utils.R: .checkClassConsistency() generalised with field + object_name args (handles classes returned under different element names, e.g. class.x, class.index)

Tests

• New test-perf- for cor, corTest, cov, kurtosis, meanSdGp, quantileMean, skewness, summary, var
• Smoke tests: added wrong-class checks; updated mean/var/quantileMean expectations
• Test expectations in tests/testthat/smk_expt-results/ regenerated because ds.mean no longer returns validity message
• Removed orphaned test-arg- checks (moved to smoke); regenerated ds.mean fixtures

Other

• Updated dsBase_7.0.0-permissive.tar.gz for CI