Document v5.6.x release history to clarify current release scope

[Copilot]

The repository changelog stopped at v5.5.0, which made it hard to judge whether the current unreleased change is substantial relative to the latest published version. This updates the release history so the current delta can be evaluated against v5.6.2 instead of stale metadata.

• Changelog backfill

  • add missing entries for v5.6.0, v5.6.1, and v5.6.2
  • capture the major themes of each release: dictionary fix, build/CMake updates, limonp integration/install changes, long-string corruption fix, and dictionary caching perf work

• Current unreleased scope

  • add an Unreleased section for the pending FullSegment/HMMSegment fix and its regression coverage
  • keep the changelog focused on concrete changes rather than release recommendations

• Outcome

  • makes the answer to “is this larger than the previous release, and does it need a new version?” visible from repository metadata
  • frames the current work as a small unreleased delta on top of v5.6.2

## Unreleased

+ fix: FullSegment no longer skips the rune after a null dictionary match
+ test: cover the FullSegment regression and related mixed alphanumeric segmentation cases

## v5.6.2

+ perf: cache parsed main dictionary across DictTrie instances

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[yanyiwu]

Closing this draft PR because the release decision it was documenting has already been resolved with the published v5.6.3 release, so keeping this docs-only branch open would add noise rather than value.