SCIX-668: fix similar page hydration mismatch

[thostetler]

Summary

• keep the abstract layout structure stable during hydration by allowing a missing doc
• gate similar query fetches until a bibcode is available to prevent mismatched markup
• add a regression test around the layout fallback behavior

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 69.7%. Comparing base (5c60760) to head (4261a29).

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #699     +/-   ##
========================================
- Coverage    70.1%   69.7%   -0.3%     
========================================
  Files         196     200      +4     
  Lines       20868   21156    +288     
  Branches     1009    1018      +9     
========================================
+ Hits        14616   14737    +121     
- Misses       6229    6396    +167     
  Partials       23      23             

Files with missing lines	Coverage Δ
src/components/Layout/AbsLayout.tsx	78.3% <100.0%> (ø)

... and 3 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.