Raise src/core/room-protocol.ts mutation score to 80%

[Mearman]

Baseline (measured against the current post-split code, agent-comms CI run 34813979900): 69.57% (119 killed, 9 timeout, 56 survived, out of 184 covered mutants).

Trace every surviving/timeout mutant against the real call graph: a genuine gap gets a real regression test; a provably equivalent/dead mutant gets documented reasoning, not a contrived test. Target the repo's own configured thresholds.high: 80 in stryker.config.ts.

Sub-issue of #64.