Prepare Gloss 0.8.4 release

[SunChJ]

Summary

• bump the App version from 0.8.3 (11) to 0.8.4 (12)
• add complete 0.8.4 release notes while preserving the 0.8.3 release-candidate history
• document that 0.8.4 is the first public build after 0.8.2
• align release guidance with the three required credentials and the Chrome-only ad-hoc/Homebrew artifact
• keep upgrade-version values in tests unchanged because they are behavior fixtures rather than the running App version

Release path

After this PR is merged, an annotated v0.8.4 tag must be created on the merged main commit and pushed. The tag workflow will build the arm64 and x86_64 App archives, publish the signed schema-v2 release metadata to SunChJ/gloss-releases, and dispatch the verified Homebrew cask update. The tap needs no additional source change.

Validation

• swift test: 290 tests, 5 conditionally skipped, 0 failures
• swift build -c release: all release products built
• Info.plist/version/release-note contract, plist, shell syntax, workflow YAML, diff, and changed-Swift lint checks
• deterministic 0.8.4 manifest, checksums, cask, public URLs, and gloss-cli binding closure

Full App packaging remains covered by PR/tag CI because the private browser-extension checkout is not present in the local workspace.