perf(render): reuse the viewer membership buffer

[TypeThe0ry]

Summary

• build the sorted viewer-membership buffer once per render snapshot
• reuse that buffer by temporarily removing and restoring one seated viewer UUID
• replace the online-viewer set with a UUID-to-existing-serialized-viewer index for fast membership checks without duplicating serialized values
• preserve exact ordering, empty-seat, offline-seat, duplicate-seat, and immutable exclusion-list behavior
• keep the change isolated to one production file

Measurement

The protected performance-snapshot JMH profile is authoritative at 4, 32, and 128 viewers. This PR must remain unmerged unless GitHub reports PASS_OPTIMIZED, including the required 128-viewer time metric.

Validation

No Gradle, JMH, tests, server, or client workload was run locally. Exact snapshot behavior tests, all platform builds, and performance measurement are GitHub-only.

[TypeThe0ry]

Rebased onto post-#80 dev (cba64e8653d6f597e848ccbb7bf92d84bd3c5f13). New immutable head: cf33b9e75fed7d6af077fddf224b5a452848bcb1. The earlier run is superseded; exact snapshot behavior and the protected 4/32/128-viewer A/B profile will rerun on this head.

[TypeThe0ry]

Post-merge isolated A/B against 0ebc5fc5242541dc18439b570c84dcce4d061f14 completed in run https://github.com/EllanServer/MahjongEngine/actions/runs/29495681881. The protected decision is FAIL_REGRESSION_OR_BEHAVIOR. Revert PR #88 removes exactly merge commit db0a5e2efb27b438afbe215ea885fab31f3b4f73 while retaining the other merged performance changes.