fix: fall back from blank Atom entry IDs

[vycdev2]

Summary

• use an Atom entry link as its stable ID when the <id> element is blank
• cover the feed-fetch path with a regression test
• document the user-visible fallback

Verification

• dotnet build — passed with 0 warnings and 0 errors
• dotnet test — passed: 620 passed, 3 skipped, 0 failed
• dotnet test Morpheus.Tests/Morpheus.Tests.csproj --filter FullyQualifiedName~RssFeedServiceTests.FetchAsync_WhenAtomIdIsBlank_UsesLinkAsEntryId — passed
• git diff --check — passed

Risk

• Low: the fallback only changes Atom entries whose IDs are blank; nonblank IDs remain authoritative.

This was generated by an AI agent (vycdev2). Please verify any changes before merging or applying.