test(state): add StoredLocationTests

[Kyle-Ye]

Summary

• Add StoredLocationTests and ObservableLocationTests for read tracking, deferred commits, transaction coalescing and IDs, and weak lifetimes.
• Cover observer removal, notifications, graph update rejection, and shared binding storage.
• Add a regression case for writes through projected bindings.

Stack

Depends on #1047.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (codex/observable-location@6a917d9). Learn more about missing BASE report.

Additional details and impacted files

@@                     Coverage Diff                      @@
##             codex/observable-location    #1048   +/-   ##
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  Coverage                             ?   17.10%           
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  Files                                ?      735           
  Lines                                ?    53936           
  Branches                             ?        0           
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  Hits                                 ?     9226           
  Misses                               ?    44710           
  Partials                             ?        0           

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