Fix linux-arm RID: rename NuGet package to linux-arm64, keep shim for compatibility

[shimat]

Summary

Fixes #1870.

The OpenCvSharp4.runtime.linux-arm NuGet package placed native binaries under
runtimes/linux-arm/native/, but the CI has always built on ARM64 hardware
(ubuntu-24.04-arm) and tested with --runtime linux-arm64. In the .NET RID graph,
linux-arm (32-bit ARMv7) and linux-arm64 (64-bit AArch64) are siblings, not
parent/child, so .NET never automatically resolves runtimes/linux-arm/native/ when
running on linux-arm64 targets — the native library was silently not copied.

Changes

• nuget/OpenCvSharp4.runtime.linux-arm64.csproj — New package with the correct runtimes/linux-arm64/native/ path.
• nuget/OpenCvSharp4.runtime.linux-arm.csproj — Converted to a compatibility shim: native .so removed, replaced with a PackageReference dependency on linux-arm64. Existing users who reference the old package name will automatically get the real library.
• .github/workflows/linux-arm.yml → linux-arm64.yml — Renamed and updated to pack both the new linux-arm64 package and the shim. Artifact name changed to artifacts_linux_arm64.
• .github/workflows/publish_nuget.yml — Updated workflow and artifact references.
• nuget/README.runtime.md / README.md — Package table updated; deprecation note added for linux-arm.

Reviewer notes

• The shim package carries a minimum-version dependency on linux-arm64 at the same version, so existing projects require no code changes — NuGet will resolve the correct native library automatically.
• The OpenCV cache key in the workflow now hashes linux-arm64.yml instead of linux-arm.yml; the existing cache will be invalidated once on the first run after merge, which is expected and harmless.

Summary by CodeRabbit

• New Features

  • Added native runtime support for Linux ARM64 (AArch64).

• Documentation

  • Announced the new Linux ARM64 runtime package and clarified that the prior Linux ARM package is retained as a deprecated compatibility shim redirecting to the ARM64 package; guidance updated to reference the ARM64 package for new projects.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

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Reviewing files that changed from the base of the PR and between 5f0c663 and d3221c3.

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• nuget/README.runtime.md

💤 Files with no reviewable changes (1)

• nuget/README.runtime.md

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📝 Walkthrough

Walkthrough

This PR adds a new OpenCvSharp4.runtime.linux-arm64 NuGet package, converts the existing linux-arm package into a deprecated compatibility shim that references linux-arm64, updates CI workflows to build/publish ARM64 artifacts and cache keys, and updates README documentation accordingly.

Changes

Linux ARM64 Package Migration

Layer / File(s)	Summary
New Linux ARM64 NuGet Package nuget/OpenCvSharp4.runtime.linux-arm64.csproj	New .csproj targeting netstandard2.0/netstandard2.1/netcoreapp3.1 with package metadata and native asset declarations; packages libOpenCvSharpExtern.so into runtimes/linux-arm64/native/ plus icon and README.
Linux ARM Compatibility Shim nuget/OpenCvSharp4.runtime.linux-arm.csproj	Updates package metadata to mark it as a deprecated compatibility shim, removes the ARM native libOpenCvSharpExtern.so from packaging, and adds a PackageReference to OpenCvSharp4.runtime.linux-arm64 (versioned by $(Version)).
Build and Publish Workflows .github/workflows/linux-arm64.yml, .github/workflows/publish_nuget.yml	Workflow name changed to "Linux ARM64"; OpenCV cache keys updated to reference linux-arm64.yml; NuGet packaging script computes a date-based version, runs dotnet pack for both linux-arm64 and linux-arm projects into artifacts_arm64, uploads artifact as artifacts_linux_arm64; publish workflow downloads artifacts_linux_arm64.
Documentation Updates README.md, nuget/README.runtime.md	Adds OpenCvSharp4.runtime.linux-arm64 entry to native bindings table and marks OpenCvSharp4.runtime.linux-arm as a deprecated compatibility shim; updates notes to explain the rename/shim behavior.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Possibly related PRs

• shimat/opencvsharp#1834: Changes to OpenCV build caching key logic in workflow files, related to ARM/ARM64 cache-key updates.
• shimat/opencvsharp#1831: Related adjustments to NuGet runtime packaging and inclusion of README.runtime.md in runtime packages.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately summarizes the main change: renaming the NuGet package from linux-arm to linux-arm64 while maintaining backward compatibility through a shim package.
Linked Issues check	✅ Passed	All objectives from issue #1870 are fully met: package RID changed to linux-arm64, runtime path updated to runtimes/linux-arm64/native/, compatibility shim provided, and CI/workflows updated accordingly.
Out of Scope Changes check	✅ Passed	All changes are directly scoped to the linked issue #1870: NuGet project files, CI workflows, documentation updates, and README modifications—no unrelated changes detected.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/linux-arm-rid

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@nuget/README.runtime.md`:
- Around line 29-31: Remove the empty line between the two consecutive
blockquote notes in the README.runtime.md so the two "Note:" blockquotes are
adjacent (this resolves markdownlint MD028); specifically, delete the blank line
separating the blockquote mentioning OpenCvSharp4.official.runtime.* and the
blockquote mentioning OpenCvSharp4.runtime.linux-arm ->
OpenCvSharp4.runtime.linux-arm64 so both blockquotes form a single back-to-back
block without intervening blank lines.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

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Run ID: 37915c69-6afc-4eb1-a3c4-fa28f317385a

📥 Commits

Reviewing files that changed from the base of the PR and between f25afda and 926835d.

📒 Files selected for processing (6)

• .github/workflows/linux-arm64.yml
• .github/workflows/publish_nuget.yml
• README.md
• nuget/OpenCvSharp4.runtime.linux-arm.csproj
• nuget/OpenCvSharp4.runtime.linux-arm64.csproj
• nuget/README.runtime.md