[rostislavrotaru]

Closes #125

TESTING.md's Determinism section gains three rules:

• the code's own timers (grace periods, debounces, intervals, timeouts) that decide which path runs are set by the test or run on a scoped mock clock;
• each suite's hang limit is one value with its reason, at least three times its slowest run on any target platform from main's runs, never raised to pass a slow run;
• a test that reaches its limit is named in the run's output.

Agent: LEAD (lead)
Agent-Model: Anthropic Opus 5.5, High