Create WebGL demo build lane for external playtesters

[JOY]

Summary

Prepare a browser-playable demo lane so external testers can try the vertical slice without installing Unity or a native client.

Scope

• Verify Unity WebGL build feasibility with current packages and networking assumptions.
• Document build settings, limitations, and backend endpoint configuration.
• Add a local or CI build command once feasible.
• Track blockers separately if Fusion, asset packages, or browser networking prevent the lane.

Acceptance Criteria

• Documentation states whether WebGL is feasible for the current vertical slice.
• A prototype WebGL build can be produced or blockers are captured with issue links.
• The build does not expose provider keys or privileged backend secrets.
• Networking and auth limitations are documented clearly.

References

• ROADMAP.md
• docs/ARCHITECTURE.md

[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.