fix(browser): re-read document.referrer when buffered value is empty

[abueide]

Summary

On iOS Safari, document.referrer is not yet populated when the snippet captures BufferedPageContext during early script execution. This causes context.page.referrer to be empty in all Segment calls for the session.

Fix: re-read document.referrer in popPageContext() when the buffered value is empty.

Chrome is unaffected — it populates document.referrer before scripts execute.

Test plan

• Unit tests for popPageContext() referrer re-read (fail without fix, pass with)

🤖 Generated with Claude Code

[changeset-bot]

⚠️ No Changeset found

Latest commit: 5a7bbae

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.33%. Comparing base (8f10626) to head (5a7bbae).
⚠️ Report is 7 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1354      +/-   ##
==========================================
+ Coverage   91.32%   91.33%   +0.01%     
==========================================
  Files         163      163              
  Lines        4393     4399       +6     
  Branches     1052     1056       +4     
==========================================
+ Hits         4012     4018       +6     
  Misses        381      381              

Flag	Coverage Δ
browser	92.24% <100.00%> (+0.01%)	⬆️
core	90.16% <ø> (ø)
node	88.52% <ø> (ø)

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