Port upstream AssemblyStore reader changes: v4/CoreCLR format, _assembly_store symbol, index-entry sizing (next major)

[jamescrosswell]

Tracking issue for upstream dotnet/android AssemblyStore reader changes that our vendored copy in src/Sentry.Android.AssemblyReader/ does not yet implement. Split out from #5451 (the watch-upstream alert for the file move). These are targeted at the next major release.

Compression (LZ4 → Zstandard) and decompressed-file caching are tracked separately in #5346 and are out of scope here.

Background

Our vendored parser is already ahead of the ATTRIBUTION.txt baseline: it tracks upstream through 64018e13 and hand-ports the v3 format (dotnet/android#10249). Comparing upstream from there to commit f1aecf9, the following logic is new upstream and missing from our copy.

Gaps to port

• v4 / CoreCLR store format. Upstream StoreReader_V2.cs now accepts 0x_0000004 versions and reads a new content_id field in the header (present when format number >= 4). We accept only v2/v3, so a v4 store is rejected. This is the highest-value item — .NET Android's move to CoreCLR is what produces v4 stores.
• _assembly_store ELF dynamic-symbol payload discovery. Utils.FindELFPayloadOffsetAndSize now locates the payload via a _assembly_store symbol in .dynsym (with a new ELFPayloadError.InvalidPayloadSymbol), in addition to the named payload section. Ours only does FindELFPayloadSectionOffsetAndSize.
• Variable index-entry sizing. Upstream derives entry size from index_size / index_entry_count (GetIndexEntrySize()) and supports the V2 index-entry layouts, rather than branching on Is64Bit with fixed sizes. Ties into v4 and hardens against corrupt or future stores.

Nice-to-have robustness that came with the same commits: checked offset arithmetic, EndOfStreamException on short reads, and ArrayPool buffer reuse in ReadEntryImageData.

Housekeeping

• Refresh src/Sentry.Android.AssemblyReader/V2/ATTRIBUTION.txt — it still cites baseline 5ebcb1dd, but the code has moved to 64018e13 + local v3.

Refs

• Watch-upstream alert: Upstream change detected: dotnet/android tools/assembly-store-reader-mk2 @ f1aecf9 #5451
• Compression/caching (separate): Replace LZ4 compression with Zstandard (next major): dotnet/android tools/assembly-store-reader-mk2 @ 2e30614 #5346
• Upstream PRs: dotnet/android#12104 (shared v1/v2/v3 + LZ4/Zstd layer), dotnet/android#12108 (file-app move)

[linear-code]

NET-777