Braze Performance fix from 0.18.6 — actually shipping this time

What shipped

• •Headline metrics for canvases now correctly show sent / delivered / unique opens (with open_rate) / unique clicks (with click_rate) / unsubscribes / bounces (hard + soft, broken out) — pulled from the step-level rollup, not from total_stats.entries which can be 0 for scheduled audiences. Conversions stay in the response as a secondary metric.
• •Triggered canvases continue to work exactly as before — the rollup is computed from the same step-level data the API already returned, the field set is just richer.
• •When a canvas has message steps but the Braze API returns no step metrics (the scheduled-audience case), the response now carries a warnings array explaining what happened. No more wrong-but-plausible zeros.
• •total_entries is preserved in a debug block on each canvas so you can still see the underlying entry count — it's just no longer the headline.
• •Performance summary now reports total_canvas_sends alongside total_canvas_entries so the aggregate row matches what an operator would expect.
• •Three new tests cover the regression paths: triggered canvas with step rollup, scheduled-blast with entries=0 + populated step rollup, and message-steps-but-empty-step_stats emitting a warning.