Replace 80% size threshold with count-first chain-to-full fallback heuristic

[JusterZhu]

Problem

The current DownloadPlanBuilder uses a single threshold: when the combined download size of all chain (delta) packages reaches 80% of the best full package size, it switches to the full package, discarding all chain packages.

This 80% threshold has several issues:

1. Ignores package count. A user with 2 large chain packages close to full size would overpay (e.g., 850 MB chain vs 1 GB full — user pays +150 MB for no reason). Conversely, 20 small chain packages well below the size threshold would still apply — creating 20 points of failure.

2. Runtime fallback already exists. Each chain package is already paired with a pre-downloaded FallbackFull. If a chain patch fails at runtime, the system automatically falls back to the full package. An overly eager download-stage switch is unnecessary.

3. Absolute overpay can be huge. 20% of a 2 GB full package is 400 MB — a significant extra download for users on metered/slow connections.

Solution

Replace with a count-first heuristic:

Condition	Trigger
Chain count > MaxChainBeforeFallback (default 8)	→ full package
Combined chain size >= full package size	→ full package

This prioritizes reliability (fewer packages = fewer failure points) while the size comparison acts as a hard guardrail (never download more than necessary).

MaxChainBeforeFallback is configurable via UpdateConfiguration — set to 0 to disable count-based fallback entirely.

Changes

• UpdateConfiguration: add MaxChainBeforeFallback (default 8)
• DownloadPlanBuilder.Build(): new count-first + size guardrail logic
• ClientStrategy: plumb MaxChainBeforeFallback through
• Tests: 5 new test cases covering count threshold, size threshold, boundary, and disabled-count scenarios