fix(kill_switch): expiry boundary off-by-one (>= → >)

[levleontiev]

Bug

Line 124 of src/fairvisor/kill_switch.lua used >= when comparing against the current time:

if expires_epoch == nil or expires_epoch >= now then

This keeps a kill switch active at the exact expiry second, contrary to standard expiry semantics (JWT exp, HTTP cookies, ISO 8601 intervals) where the expiry timestamp is the first moment the resource is no longer valid.

Fix

Change >= to >:

if expires_epoch == nil or expires_epoch > now then

Tests

Added AC-13 to spec/unit/features/kill_switch.feature:

expires_at == now → result is not matched

Existing AC-5 (1s after expiry → not matched) and AC-6 (1s before → matched) continue to pass.