test: document multi-target alias watch-mode fallback (#395)

[alexander-akait]

Issue #395 reports that deleting a file resolved through a multi-target
AliasPlugin causes ENOENT until the dev server is restarted. Investigation
shows AliasPlugin itself already falls back to the next target on the
next resolve and correctly records the winning target in
fileDependencies and untried higher-priority targets in
missingDependencies; the stale path observed in practice comes from
upstream caching (webpack module graph / loader caches) that must be
invalidated by the consumer.

Add regression tests that pin this behavior down for future changes and
extend the AliasPlugin JSDoc with the watch-mode contract so consumers
know what they need to invalidate.

https://claude.ai/code/session_015JAmrjYqXwmjn6Wdc8KsD9

[linux-foundation-easycla]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.52%. Comparing base (b5259a0) to head (7f17614).
⚠️ Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #531      +/-   ##
==========================================
- Coverage   96.75%   96.52%   -0.23%     
==========================================
  Files          50       50              
  Lines        2589     2651      +62     
  Branches      788      808      +20     
==========================================
+ Hits         2505     2559      +54     
- Misses         69       77       +8     
  Partials       15       15              

Flag	Coverage Δ
integration	96.52% <ø> (-0.23%)	⬇️

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[codspeed-hq]

Merging this PR will not alter performance

✅ 58 untouched benchmarks

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_{Comparing claude/fix-enhanced-resolve-395-6BCVG (7f17614) with main (990c604)}