#21458 fix(cmux-next): surface summary-only nightly What’s New entries

Problem

Recent nightly feeds can contain validated user-facing summaries while omitting a whatsNew digest. The app then falls back to the generic “All Changes” fold, which makes the What’s New page look stale and loses the New/Improved hierarchy.

Change

• Generate a validated What's New digest from summary lines when no highlight digest is available.
• Add an app-side fallback for already-published summary-only notes, preserving categories and ignoring empty lines.
• Keep the existing embedded digest/highlight paths unchanged.

Validation

• python3 -m unittest tests/test_cmux_next_release_notes.py — 7/7 passed.
• swiftc -parse Packages/macOS/CmuxNext/Sources/CmuxNextUpdater/WhatsNew/WhatsNewSources.swift Packages/macOS/CmuxNext/Tests/CmuxNextAppTests/FeedWhatsNewSourceTests.swift — passed.
• git diff --check — passed.
• Live feed samples observed summaries with whatsNew: false on builds 3909161799401, 3908291645501, and 3907660300601.

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Summary by cubic

Fixes the What's New page falling back to the generic "All Changes" fold when a nightly build ships validated summaries but no highlight digest. Summary lines now generate a What's New digest with the New/Improved/Fixed/Security hierarchy, and existing embedded digest and highlight paths are unchanged.

New Features

• The release-notes generator produces a validated whatsNew digest from user-facing summary lines when no highlight digest exists.
• The macOS app builds a fallback What's New document from summary-only feeds, preserving categories and skipping empty lines.