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Add collections module: HashMap bucket layout, treeification, and a real ConcurrentModificationException reproduction

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Companion code for the ankurm.com post "Top 40 Java Collections Interview Questions
(HashMap Internals to Fail-Fast Iterators)". Verified against JDK 25's own HashMap
source (TREEIFY_THRESHOLD=8, UNTREEIFY_THRESHOLD=6, MIN_TREEIFY_CAPACITY=64), with
every claim backed by a committed docs/output/*.txt transcript from a real run.

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01YXCrLgRKFgCh9RHKW8xaqJ
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asmhatre
2026-10-01 04:27:27 +00:00
co-authored by Claude Sonnet 5
commit d4b3a63615
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collections/docs/output/03-bucket-layout-small-map.txt
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Real bucket layout of a 6-entry HashMap<String,Integer> (default capacity)
==========================================================================
map.size() = 6
table.length = 16 | non-empty buckets = 4
bucket[ 0] chain=2 kind=Node
bucket[ 1] chain=2 kind=Node
bucket[ 4] chain=1 kind=Node
bucket[ 5] chain=1 kind=Node
Index for each key, computed the same way HashMap.hash() does it:
apple hashCode=93029210 spread=93030097 index=(16-1)&spread=1
banana hashCode=-1396355227 spread=-1396317280 index=(16-1)&spread=0
cherry hashCode=-1361513063 spread=-1361552575 index=(16-1)&spread=1
date hashCode=3076014 spread=3075968 index=(16-1)&spread=0
egg hashCode=100357 spread=100356 index=(16-1)&spread=4
fig hashCode=101380 spread=101381 index=(16-1)&spread=5