Raise src/core/mesh-store.ts mutation score to 80%

[Mearman]

Baseline (measured against the current post-split code, agent-comms CI run 34813979900): 60.19% (43 killed, 19 timeout, 41 survived, 104 errors, out of 104 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.