perf: eliminate duplicate by_index() call in ZIP extraction

[bug-ops]

Summary

• Restructure ZipArchive::process_entry() from two by_index() calls per entry to one
• Use single by_index() with three-way branching on entry type (directory/symlink/file)
• Add ZipEntryAdapter::is_symlink_from_mode() for symlink detection from Unix mode bits
• Remove unused to_entry_type() and is_symlink() methods

Benchmark Results

Benchmark	Change	Statistical
file_count_scaling/5000	-7.1%	p < 0.05
many_small_files/1000	-4.6%	p < 0.05
file_extraction/zip_small	-11.8%	p < 0.05
large_files/100MB	noise	p = 0.71
path_validation	unchanged	p = 0.60

Test plan

• 750 tests pass (3 skipped, known zip crate symlink limitation)
• Clippy clean, zero warnings
• Documentation builds cleanly
• Security audit: validation before disk writes preserved in all branches
• No regressions in any benchmark category

Closes #52

[codecov-commenter]

Codecov Report

❌ Patch coverage is 65.27778% with 25 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
crates/exarch-core/src/formats/zip.rs	65.27%	25 Missing ⚠️

❌ Your patch status has failed because the patch coverage (65.27%) is below the target coverage (80.00%). You can increase the patch coverage or adjust the target coverage.

@@           Coverage Diff            @@
##             main      #54    +/-   ##
========================================
  Coverage   90.30%   90.31%            
========================================
  Files          62       58     -4     
  Lines       10347    10191   -156     
========================================
- Hits         9344     9204   -140     
+ Misses       1003      987    -16     

Flag	Coverage Δ
exarch-python	?

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Files with missing lines	Coverage Δ
crates/exarch-core/src/formats/zip.rs	78.79% <65.27%> (-0.57%)	⬇️

... and 4 files with indirect coverage changes

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