Give copied annotations distinct references

[calixteman]

When a viewer page was copied, each copy got its own annotationStorage entry, but newAnnotationsByPage was keyed only by source page. As a result, every output copy received all entries and reused their memoized references.

Tag each entry with its rank among output copies of the same source page. The display and worker compute this rank independently; inserted documents preserve the order of copies. Keep unextracted entries at rank -1 so shared stamp bitmaps remain available without being written. Without ranks, new annotations are applied only to the first copy.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 98.14815% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 90.05%. Comparing base (e57a464) to head (bf7c925).

Files with missing lines	Patch %	Lines
web/app.js	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #21707      +/-   ##
==========================================
+ Coverage   90.04%   90.05%   +0.01%     
==========================================
  Files         264      264              
  Lines       66894    66933      +39     
==========================================
+ Hits        60232    60275      +43     
+ Misses       6662     6658       -4     

Flag	Coverage Δ
browsertest	66.47% <23.52%> (-0.05%)	⬇️
fonttest	9.03% <ø> (ø)
integrationtest	69.38% <81.48%> (+0.02%)	⬆️
unittest	57.96% <76.47%> (+0.08%)	⬆️
unittestcli	56.71% <76.47%> (+0.10%)	⬆️

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[timvandermeij]

Looks good to me; thanks!