fix(ci): rename DMG artifact to ForgeCode3

[ssddOnTop]

What

Renames the release DMG from ForgeCode.dmg to ForgeCode3.dmg. Follow-up to #42, which made the name static.

Changes

• macos/scripts/common.sh — DMG_NAME default is now ForgeCode3. This drives DMG_PATH, staging temp filenames, and the manifest dmg_name field.
• .github/workflows/release.yml — the "Derive APP_VERSION" step emits DMG_NAME=ForgeCode3$suffix / dmg_file=ForgeCode3$suffix.dmg. The -unsigned suffix for PR dry runs is preserved.

Notes

• The draft-release asset cleanup jq filter uses startswith("ForgeCode"), which already matches ForgeCode3.dmg — no change needed, and previously-uploaded assets are still pruned correctly.
• Only the DMG filename changes. The app bundle stays ForgeCode.app and the DMG volume name stays ForgeCode (both driven by APP_NAME, untouched).
• Sparkle is unaffected: the appcast enclosure URL is tag-scoped and version info comes from sparkle:shortVersionString.

Verification

sh scripts/test-packaging.sh — all 18 checks pass.