Document cdidx distribution and installer guidance

[Widthdom]

Summary

• add a distribution channel matrix covering install.sh, NuGet, Windows ZIP assets, source builds, planned package managers, and package maintainer boundaries
• document NuGet global tool SDK install/runtime execution requirements, including ARM64 guidance
• document install.sh proxy, mirror, --doctor, and local mirror self-test usage in English and Japanese

Documentation and changelog

• added DISTRIBUTION.md
• updated README.md, USER_GUIDE.md, INTEGRATION_POLICY.md, COMMERCIAL_LICENSE.md, DEVELOPER_GUIDE.md, and install.sh usage comments
• added changelog.d/unreleased/1951.docs.md
• added changelog.d/unreleased/1952.docs.md
• added changelog.d/unreleased/1957.docs.md

Validation

• bash -n install.sh
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false
• git diff --check origin/main..HEAD
• adversarial review: No blocking/actionable issues found.

Notes

• dotnet test CodeIndex.sln -c Release -p:UseSharedCompilation=false reached VSTest for net8.0/net9.0 but produced no test results for more than 8 minutes while other long-running dotnet/vstest processes were present on the machine, so it was stopped and treated as inconclusive.

Fixes #1951
Fixes #1952
Fixes #1957