refactor(follow): extract merger logic into a testable method

[pamburus]

Summary

• Extract the merger thread's inline logic in follow mode into a standalone testable method, mirroring the pattern already used for segment processing
• Add 6 unit tests covering the merger logic: single line output, timestamp ordering, multi-source interleaving, out-of-order sync indicator, empty channel, and multiple lines per segment
• No logic changes — pure refactoring

Motivation

The merger logic in follow mode was an inline closure spanning ~60 lines inside a thread spawn call, making it impossible to test without OS-level file watching and threading. Extracting it into a method allows direct unit testing via channels, unblocking test coverage for upcoming changes to this code path.

[codecov]

Codecov Report

❌ Patch coverage is 96.07843% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.99%. Comparing base (5669f0a) to head (cacd126).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/app.rs	96.07%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1353      +/-   ##
==========================================
+ Coverage   89.83%   89.99%   +0.16%     
==========================================
  Files          72       72              
  Lines       12246    12258      +12     
==========================================
+ Hits        11001    11032      +31     
+ Misses       1245     1226      -19     

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