CAMEL-23693: Optimize CaseInsensitiveMap hash for ASCII keys

[gnodet]

CAMEL-23693

Summary

Optimize the CaseInsensitiveMap hash function and eliminate redundant hash computations introduced in CAMEL-23691.

• ASCII fast-path in hash: c += 32 for A-Z instead of Character.toLowerCase(Character.toUpperCase(c)) per character. Full Unicode two-step fold only for non-ASCII (c >= 128)
• Single-hash put(): compute hash once, reuse for deduplication lookup, find, and bucket insertion (was 3 separate hash computations)
• Single-hash remove(): inline chain walk with one hash computation (was 2)

Benchmark (18 typical headers, 2M iterations)

Scenario	Before	After	vs TreeMap (old impl)
put/get/containsKey/remove	9.0M ops/s	33.3M ops/s	1.2x faster
Exchange copy (create+populate+copy)	490K copies/s	1.9M copies/s	1.1x faster
Get-heavy (10 lookups/iter)	16.9M lookups/s	39.8M lookups/s	1.7x faster

Test plan

• CaseInsensitiveMapTest — 32 tests pass
• DefaultMessageHeaderTest — 38 tests pass

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🧪 CI tested the following changed modules:

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[davsclaus]

Clean performance follow-up to #23766. The ASCII fast-path (c += 32 for A-Z) is correct and produces identical hash values as the previous Character.toLowerCase(Character.toUpperCase(c)) for ASCII, with full Unicode fallback for non-ASCII. Single-hash put() and remove() eliminate redundant computations on the hot path. CI is green, existing tests cover correctness.

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