[PR #1382/486ab875 backport][3.36] fix: stabilize RSS feed GUIDs for multi-arch wheel publication

[patchback]

This is a backport of PR #1382 as merged into main (486ab87).

Summary

• RSS feed GUIDs now use PEP 427 build tags instead of timestamps, preventing duplicate entries in feed readers when platform wheels for the same build arrive at different times.
• GUIDs still change when a new build tag is introduced (the RSS feeds show stale timestamps for rebuilt packages and have a low update limit #1374 requirement).

Fixes #1381

Test plan

• Existing feed tests updated and passing
• New functional test: build tag 1 -> GUID #builds=1; add tag 2 -> GUID #builds=1,2; add new arch for tag 2 -> GUID stable
• Unit tests for _build_tag_fragment

🤖 Generated with Claude Code

Summary by CodeRabbit

• Bug Fixes
  • RSS feed entry identifiers now remain stable when additional platform-specific wheels are published for an existing release.
  • Prevented duplicate feed entries for multi-architecture builds sharing the same build tag.
  • Feed identifiers now change only when a new PEP 427 build tag is introduced.
  • Feed publication dates continue to update when additional files are published without changing the build tag.

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