#23018 cmux-next tests: AgentPaneProjectRepoTests png fixture is nonisolated (base test-target compile red)

Base red: CmuxNextAgentPaneTests doesn't compile on feat-cmux-next, so every package test run that builds the test bundle fails:

AgentPaneProjectRepoTests.swift:237:119: error: main actor-isolated static property 'png' cannot be accessed from outside of the actor
AgentPaneProjectRepoTests.swift:248:104: error: main actor-isolated static property 'png' cannot be accessed from outside of the actor

The avatar fetch closures run off the main actor and read Self.png, an immutable Data fixture. That fixture is now nonisolated.

Changelog

none

Observed / Not run

• Observed: the errors above in a fleet package-test-lane.sh run on #23003's head.
• Running now: a fleet AgentPaneProjectRepoTests run on this head.

Agent session: cosmic-gray

🤖 Generated with Claude Code

---

^{Need help on this PR? Tag @codesmith-bot with what you need.}

---

Summary by cubic

Fixes the CmuxNextAgentPaneTests test target failing to compile on feat-cmux-next, which blocked all package test bundle builds.

• Marks the png fixture as nonisolated so avatar fetch closures running off the main actor can read it.