A reason records *why* an event is a false trigger. It is optional (plain
FT flags still record no reason) and is a subtype of the FT flag: setting a
reason implies false_trigger=1, and the reason is cleared whenever FT ends
up 0 (confirm-real, clear-FT, set_false_trigger(false)). Twin propagation
carries the reason to the histogram/waveform twin alongside the FT flag.
New nullable `false_trigger_reason TEXT` column (schema + _migrate ADD
COLUMN only — not the Migration-1 rebuild). 7 tests.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YDXjZCr4RqT2U3QvMDhgzf
