[jmalloc]

Summary

Adds xsync.ChangeObservable(), which updates an xsync.Observable only if the new value differs from its current value. Readers are woken only when something actually changed.

func ChangeObservable[T comparable](o *Observable[T], v T) bool

It reports whether the value was changed. The comparison and the swap happen in the same compare-and-swap loop that Store uses, so it's atomic with any number of concurrent writers: two writers can't both see the old value and both report a change. "Change" carries the "only if different" sense on its own, so the name doesn't need an "if changed" suffix.

It's a package-level function rather than a method because it needs T to be comparable, and Observable[T any] doesn't require that. Go 1.27's method type parameters can't add a constraint to the receiver's T or tie a method's own type parameter to it, so a method would have to fall back to a runtime type assertion.

This PR also bumps the minimum Go version to v1.27.

Motivation

runkit's projection consumer stores whether this node is elected to consume each event stream, and each stream's worker restarts consumption when that value changes. Storing an unchanged value wakes the worker for nothing. Comparing before storing outside xsync isn't atomic, so this belongs here.

Testing

x/xsync has tests for changing the value, skipping an unchanged one, treating the zero value as the initial value, and concurrent writers. The concurrent test checks that the final boolean matches the parity of the calls that reported a change. It fails against a non-atomic load-compare-store.

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