Fix CMake export for external LZ4; doc vcpkg overlays

[ptahmose]

Description

This PR fixes the installed CMake package configuration for builds where experimental chunked compression is enabled with an external LZ4 package.

When libCZI is built with LIBCZI_BUILD_EXPERIMENTAL_CHUNKED_COMPRESSION=ON and LIBCZI_BUILD_PREFER_EXTERNALPACKAGE_LZ4=ON, the exported libCZI target can reference an LZ4 CMake target. The generated libCZIConfig.cmake now preserves that build-state and calls find_dependency(lz4 CONFIG REQUIRED) before importing libCZITargets.cmake.

The PR also documents vcpkg usage, including the recommended overlay-port approach for consuming experimental libCZI features, and bumps the version to 0.69.1.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Configured libCZI with the vcpkg toolchain and:

-DLIBCZI_ENABLE_INSTALL=ON
-DLIBCZI_BUILD_EXPERIMENTAL_CHUNKED_COMPRESSION=ON
-DLIBCZI_BUILD_PREFER_EXTERNALPACKAGE_LZ4=ON
-DLIBCZI_BUILD_PREFER_EXTERNALPACKAGE_ZSTD=ON
-DLIBCZI_BUILD_PREFER_EXTERNALPACKAGE_EIGEN3=ON

Verified that the generated libCZIConfig.cmake contains the expected conditional find_dependency(lz4 CONFIG REQUIRED).

Checklist:

• I followed the Contributing Guidelines.
• I did a self-review.
• I commented my code, particularly in hard-to-understand areas.
• I updated the documentation.
• I updated the version of libCZI following Versioning of libCZI depending on the type of change
  • Bug fix -> PATCH
  • New feature -> MINOR
  • Breaking change -> MAJOR
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.

[Copilot]

Pull request overview

This PR fixes libCZI’s installed CMake package configuration so downstream find_package(libCZI CONFIG REQUIRED) correctly resolves an external LZ4 dependency when experimental chunked compression is enabled, and it updates the docs to better describe vcpkg usage (including overlay ports). It also bumps the project version and records the change in the version history.

Changes:

• Update libCZIConfig.cmake.in to conditionally find_dependency(lz4 CONFIG REQUIRED) when the installed/exported targets may reference LZ4::....
• Expand build documentation with a dedicated vcpkg section, including recommended overlay-port workflow for experimental features.
• Bump version to 0.69.1 and add an entry to the version history.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
docs/source/pages/version_history.md	Adds the 0.69.1 entry documenting the fix and vcpkg overlay-port docs.
docs/source/pages/building.rst	Adds/expands vcpkg guidance and documents overlay ports for experimental feature consumption.
CMakeLists.txt	Bumps project version to 0.69.1.
cmake/libCZIConfig.cmake.in	Preserves build flags for chunked compression / external LZ4 and conditionally finds lz4 before importing targets.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.91%. Comparing base (3ec0807) to head (0ded1f2).

Additional details and impacted files

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  Coverage   67.91%   67.91%           
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  Files          99       99           
  Lines       13467    13467           
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  Hits         9146     9146           
  Misses       4321     4321           

Flag	Coverage Δ
windows-latest	67.91% <ø> (ø)

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