chore(sync-service): instrument snapshot query timing

[KyleAMathews]

Summary

• add a top-level snapshot query span around SnapshotQuery.execute_for_shape/4
• record checkout wait, setup, query, and total durations for snapshot-backed requests
• emit a telemetry event tagged with query_reason so subset, move-in, and initial snapshot paths can be separated in analysis

Verification

• mix compile in packages/sync-service
• mix test test/electric/shape_cache_test.exs in packages/sync-service (real-db cases fail here because local Postgres on localhost:54321 is unavailable in this environment)

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.67%. Comparing base (c740930) to head (cdea4ee).
⚠️ Report is 19 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4110   +/-   ##
=======================================
  Coverage   88.67%   88.67%           
=======================================
  Files          25       25           
  Lines        2438     2438           
  Branches      615      611    -4     
=======================================
  Hits         2162     2162           
  Misses        274      274           
  Partials        2        2           

Flag	Coverage Δ
packages/experimental	87.73% <ø> (ø)
packages/react-hooks	86.48% <ø> (ø)
packages/start	82.83% <ø> (ø)
packages/typescript-client	93.81% <ø> (ø)
packages/y-electric	56.05% <ø> (ø)
typescript	88.67% <ø> (ø)
unit-tests	88.67% <ø> (ø)

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