PackageType enum migration + chain/full fallback mechanism

[JusterZhu]

Closes #524

Summary

Migrate PackageType.Chain from 0→1, implement chain→full fallback on failure, and size-based full-package switching when chain total ≥ 80% of full. Remove all CVP concepts.

See issue #524 for full details.

[Copilot]

Pull request overview

This PR updates the update/patch pipeline to support a migrated PackageType model (adding Unspecified=0, shifting Chain to 1) and introduces a “chain → full” fallback flow, including a size-based switch to prefer full packages when chain downloads are close in size.

Changes:

• Add PackageType + fallback-full metadata to version/assets and propagate it through planning, download, and apply pipelines.
• Implement chain-apply failure fallback to a matching full package, and skip patch middleware for full packages.
• Add size-based switching (chain total ≥ 80% of full) and remove CVP concepts.

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
src/c#/GeneralUpdate.Core/Strategy/WindowsStrategy.cs	Skip PatchMiddleware for full packages based on PackageType in pipeline context.
src/c#/GeneralUpdate.Core/Strategy/MacStrategy.cs	Same as Windows: conditional patching based on PackageType.
src/c#/GeneralUpdate.Core/Strategy/LinuxStrategy.cs	Same as Windows: conditional patching based on PackageType.
src/c#/GeneralUpdate.Core/Strategy/ClientStrategy.cs	Downloads fallback-full assets alongside primary assets and propagates fallback metadata.
src/c#/GeneralUpdate.Core/Strategy/AbstractStrategy.cs	Adds chain-failure fallback execution path and injects PackageType into pipeline context.
src/c#/GeneralUpdate.Core/Pipeline/CompressMiddleware.cs	Extract full packages directly to install directory regardless of patch settings.
src/c#/GeneralUpdate.Core/Download/Sources/HttpDownloadSource.cs	Maps PackageType from VersionEntry to DownloadAsset.
src/c#/GeneralUpdate.Core/Download/Models/DownloadAsset.cs	Adds PackageType + fallback full fields; adds FallbackFulls to DownloadPlan.
src/c#/GeneralUpdate.Core/Download/DownloadPlanBuilder.cs	Implements chain vs full selection, and attaches fallback-full metadata to chain assets.
src/c#/GeneralUpdate.Core/Configuration/VersionEntry.cs	Replaces CVP fields with PackageType + fallback full metadata.
src/c#/GeneralUpdate.Core/Configuration/PackageType.cs	Introduces PackageType enum with Unspecified=0, Chain=1, Full=2.

Comments suppressed due to low confidence (1)

src/c#/GeneralUpdate.Core/Strategy/ClientStrategy.cs:616

• Fallback full assets are merged into the download plan, but any fallback download failure increments FailedCount and aborts the apply phase even if all primary assets downloaded successfully. Fallback downloads should be treated as optional until actually needed.

            // Merge fallback full packages into the download plan so they are
            // available locally if a chain package fails and needs fallback.
            var allAssets = plan.Assets.Concat(plan.FallbackFulls)
                                      .GroupBy(a => a.Url)
                                      .Select(g => g.First())
                                      .ToList();
            var mergedPlan = new Download.Models.DownloadPlan(allAssets, plan.IsForcibly);

            GeneralTracer.Info($"ClientStrategy: downloading {mergedPlan.Assets.Count} asset(s) ({plan.Assets.Count} primary + {plan.FallbackFulls.Count} fallback).");
            var downloadReport = await ExecuteDownloadAsync(mergedPlan).ConfigureAwait(false);

            if (downloadReport.FailedCount > 0)
            {
                var failDetails = string.Join(", ",
                    downloadReport.Results.Where(r => !r.Success)
                        .Select(r => $"{r.Asset.Name}: {r.ErrorMessage}"));
                GeneralTracer.Error($"ClientStrategy: {downloadReport.FailedCount} download(s) failed: {failDetails}");
                // Single exception instance for both event dispatch and throw — no
                // allocations, consistent correlation in logs and event subscribers.
                var ex = new InvalidOperationException(
                    $"{downloadReport.FailedCount} download(s) failed. Aborting apply phase.");
                EventManager.Instance.Dispatch(this, new ExceptionEventArgs(ex, "Download failures detected."));
                // Throw so CVP fallback can retry with chain packages.
                throw ex;
            }

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