List a signal's occurrence events on one ticker

Per-bar booleans (`history`) are useful for historical analysis; per-occurrence records are what you want for chart overlays and dashboards. This endpoint returns each *firing* of a signal on a ticker as a single row. For **state**-style signals (`above_sma_50`, `in_uptrend`, `at_52w_high`, …) each row is a true-window: `started_at` is when the condition flipped false→true, `ended_at` is when it flipped back (or null if it's still true). `start_price` and `end_price` carry the price at the open/close of the window. For **event**-style signals (`golden_cross_today`, `gap_up`, `breakout`, …) each row is a point firing: `started_at` is when the event fired and `ended_at` is the same value. `start_price` is the price at that bar. Custom (expression) signals are returned in the state shape — the expression is evaluated per bar and contiguous true runs are collapsed into windows.