Fix Unity Package Manager path error on 6000.5.0b9

[JOY]

Problem

Unity 6000.5.0b9 can open the SECOND SPAWN project far enough to start Package Manager, but package resolution fails before C# compilation.

Observed log line from Unity/Logs/Editor.log:

[Package Manager] The "path" argument must be of type string. Received undefined
Failed to resolve packages: The "path" argument must be of type string. Received undefined. No packages loaded.

Current impact

• Editor appears stuck around package/script startup.
• Batch mode exits code 1 before script compilation.
• Unity Play Mode smoke cannot be trusted until this is fixed.
• No error CS compiler errors were emitted in the batch log during the latest check.

Work already done

• Updated repo baseline metadata from Unity 6000.5.0b8 to 6000.5.0b9 in PR chore: update Unity b9 baseline metadata #121.
• Normalized FunPlay package URL and lock file in PR chore: normalize FunPlay package path #120.
• Confirmed Unity still reports the Package Manager path error after opening b9 directly.
• Closed stale b8/b9 Unity and UPM processes after testing so the workstation is not left spinning.

Next diagnostic ideas

• Check whether this is a Unity beta Package Manager regression in 6000.5.0b9.
• Try launching the project from Unity Hub after pulling PR chore: update Unity b9 baseline metadata #121 so Hub reads ProjectVersion.txt as b9.
• Compare with a clean b9 project after clearing Unity Package Manager caches.
• If clean b9 also fails, repair or reinstall the b9 editor or move back to the last known working beta only with JOY approval.
• If only SECOND SPAWN fails, bisect Unity/Packages and project package settings.

Guardrails

• Do not remove or downgrade Unity packages without JOY approval.
• Do not claim Unity smoke passed until Package Manager resolves packages and C# compilation reaches real compiler output.

[JOY]

Implementation packet added in PR #188: docs/design/45-alpha-backlog-execution-packets.md.

Use that packet as the build contract for this issue: goal, minimum data contract, Unity/backend surface, verification, and cut line. This comment does not close the issue; implementation still needs its own PR and runtime evidence where applicable.

[JOY]

Resolved by restarting Unity according to live local verification from JOY. No package manifest or package-lock changes were needed, and no Unity package was removed or downgraded. If the Package Manager path error returns, reopen with the latest Editor or batchmode log.