fix: performance API docs, trend is null when the previous period is 0 - #151

What changed

api-reference/performance.mdx: trend object table (trend is a number or null, new previous), the zero-previous rule, the rounding-to-stable rule, example JSON (values consistent: clicks 60 vs 52 gives 15.4 up; previous-0 metrics give null up), and last_updated.

Why

cms#510 (merge 2bfb463, tag v3.16.114, live) changes the period trend: growth from a previous value of 0 has no percentage (null, was 100), and each trend object now carries previous.

How verified

• affiliate-cms v3.16.114:src/utils/trend.ts periodTrend: previous 0 gives percentage: current === 0 ? 0 : null, direction by sign; otherwise the change is rounded to 0.1 first and the direction comes from the rounded change.
• v3.16.114:src/api/affiliate-tracking/controllers/affiliate-tracking.ts:817-821: calculateTrend returns { trend: percentage, trendDirection: direction, previous } for all 7 trends (route /tracking/performance/:programId, routes file line 30).
• No other docs mention trendDirection (grep over content and public).
• Build, check-agent-surface, check-docs-host and check-docs-host-http pass locally. No media.

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