feat(core): replace 80% size threshold with count-first chain-to-full fallback

[JusterZhu]

Closes #530

Summary

Replaces the single 80%-of-full-size threshold in DownloadPlanBuilder with a count-first heuristic for choosing between chain packages and a full replacement package.

Before

// 80% of full size threshold
var threshold = (long)(bestFull.Size * 0.8);
if (chainTotal >= threshold) → full package

After

// Count-first: too many chain packages → full
if (maxChainBeforeFallback > 0 && chainCount > maxChainBeforeFallback) → full package

// Size guardrail: chain is larger than full → full
if (chainTotal >= bestFull.Size) → full package

Why

• Package count matters. 20 small chain packages have 20 points of failure (download, hash verify, patch apply). Fewer packages = more reliable updates.
• Runtime fallback already exists. Chain packages are paired with FallbackFull — a failed patch falls back automatically. We don't need an eager 80% threshold at download time.
• No absolute overpay. Old logic could make users download 400 MB extra (20% of 2 GB). New logic only switches when it genuinely saves data or reliability demands it.
• Configurable. MaxChainBeforeFallback defaults to 8 in UpdateConfiguration, set to 0 to disable count-based fallback.

Changes

File	Change
UpdateConfiguration.cs	+ MaxChainBeforeFallback property (default 8)
DownloadPlanBuilder.cs	New count+size heuristic, new parameter
ClientStrategy.cs	Pass MaxChainBeforeFallback to Build
DownloadPlanBuilderTests.cs	5 new test cases

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