fix: report a runtime property chain's initial value once when a link emits on subscribe

[glennawatson]

Summary

A runtime property chain reports its initial value once, even when a link has no change notification.

• WhenAnyValueUnsafe, WhenChanged and the other reflection-based observations report one initial value. This holds when a link in the chain is a plain property with no change notification.
• BindUnsafe does not write back to the view model when the binding is created. This holds when the view raises no change notification of its own.

Why

A link that emits as soon as you subscribe produced a second initial value.

• The fallback provider for a property with no change notification emits once on subscribe. The chain also reads the value itself right after it subscribes. A plain property in the middle of a chain re-attached the rest of the chain twice. A plain property at the end of a chain sent its value twice.
• isDistinct did not collapse the repeat. With skipInitial, the chain still let the initial value through.
• A two-way binding saw the second value on the view side and wrote it back to the view model.

Closes #160

Breaking changes

None

How this was verified

• Tests cover a plain property in the middle of a chain and at the end, with and without skipInitial. They also cover a two-way binding through a view that raises no notification.
• Ran the Linux examples suite and the Windows examples suite on a Windows guest.

Notes for the reviewer

The whole fix is in ExpressionChainSink.cs.

• Each link sets a flag while it subscribes. A notification that arrives while the flag is set is ignored, because the chain reads the same value straight after.
• The flag is only read under the chain's lock. Only a notification raised from inside Subscribe on the same thread can see it. Notifications from other threads still wait on the lock and are delivered as before.
• The fix covers any provider that emits on subscribe, not only the fallback one.

Checklist

• I have read the Contribute guide
• The PR title follows Conventional Commits
• Tests cover this change, or the summary says why they do not
• New or changed public API has XML documentation

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.23%. Comparing base (760b967) to head (55f58a5).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #162   +/-   ##
=======================================
  Coverage   99.23%   99.23%           
=======================================
  Files         396      396           
  Lines       14467    14469    +2     
  Branches     2065     2065           
=======================================
+ Hits        14357    14359    +2     
  Misses        110      110           

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud