Avoid blocking WithLatestFrom value updates

[glopesdev]

WithLatestFrom pairs each source element with the latest value from a second sequence. It was initially built on top of CombineLatest, which serializes both sequences and downstream delivery behind a single lock. With that one lock held for the duration of the downstream OnNext, an incoming element on the value sequence could not refresh the cached latest value until the call returned, so a slow downstream observer would stall the value sequence for as long as it ran. This is common when software-timestamping a data sequence against a high-rate clock, where a slow consumer of the timestamped element stalls the shared clock sequence, blocking its producer so the device input buffer can overflow.

This change reimplements the operator on the two-lock strategy from the current WithLatestFrom in dotnet/reactive. One lock serializes downstream delivery, and a separate lock guards only the cached latest value. The value sequence now updates the cached value under its own lock without waiting on downstream work, so a slow observer delays only further source elements, not value updates.

Validation

Observable behavior is otherwise unchanged. The value sequence is subscribed before the source, so a synchronously available value is ready when the source emits immediately. The source observer forwards completion directly, so the result sequence completes with the source and no longer needs the TakeUntil clause the previous composition added to fix #670. A completed value sequence retains its last value, and an error on either sequence faults downstream.

WithLatestFromTests was added to guard the behavior-preservation dimensions against regression: pairing with the latest value, source completion, value-sequence completion retaining the last value, dropping a source element that arrives before any value, faulting on either sequence, and synchronous value availability.

Closes #2600