feat: Support Zstandard-compressed assemblies on Android (.NET 11)

[jamescrosswell]

Stacked on #5574 — review that first; this PR's diff is only the Zstandard change.

.NET 11 compresses assemblies in the Android assembly store with Zstandard instead of LZ4 (dotnet/android#11730). The 12-byte header (magic / descriptor index / uncompressed length) is unchanged; only the magic (XALZ → XAZS) and the codec differ.

Closes #5346

Summary

• ArchiveUtils.TryDecompressLZ4 → TryDecompress, which recognises both magics and dispatches to LZ4 or Zstandard.
• Zstandard uses the BCL's System.IO.Compression.ZstandardDecoder on net11.0 — no new package dependency.
• On net10.0 a XAZS payload throws NotSupportedException with a clear message instead of handing compressed bytes to PEReader (which failed with an obscure BadImageFormatException). Only .NET 11 apps produce XAZS, and those resolve the net11.0 build, so this is defensive.
• New ArchiveUtilsTests round-trip a real PE image through uncompressed, LZ4, Zstandard and MemorySlice inputs.

Notes

• LZ4 support stays: compression format is per-assembly and .NET 10 stores still use XALZ. So K4os.Compression.LZ4 can't be dropped, contrary to the original motivation in Add support for Zstandard: dotnet/android tools/assembly-store-reader-mk2 @ 2e30614 #5346.
• The "re-use cached copies of decompressed files" change referenced in Add support for Zstandard: dotnet/android tools/assembly-store-reader-mk2 @ 2e30614 #5346 (dotnet/android#11967) is an opt-in on-device cache used by the native runtime, not something an APK reader needs, so it isn't ported.
• The APK-based AndroidAssemblyReaderTests stay skipped on net11.0. They also need variable index-entry sizing from Port upstream AssemblyStore reader changes: v4/CoreCLR format, _assembly_store symbol, index-entry sizing (next major) #5454 (.NET 11 stores use 9-byte index entries on 64-bit ABIs). With that hacked in locally, every compressed .NET 11 APK case reads assemblies successfully with this change.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.79%. Comparing base (4ee11d6) to head (082cf3e).

Additional details and impacted files

@@                    Coverage Diff                    @@
##           feat/assemblystore-v4    #5575      +/-   ##
=========================================================
+ Coverage                  74.74%   74.79%   +0.05%     
=========================================================
  Files                        515      515              
  Lines                      18896    18902       +6     
  Branches                    3686     3689       +3     
=========================================================
+ Hits                       14123    14138      +15     
+ Misses                      3893     3887       -6     
+ Partials                     880      877       -3     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.