chore(trends): cover weekday bucket alignment

[lordspline]

Problem

Trends contributors lacked regression coverage proving weekday exclusions preserve populated days after the most recent weekend.

PostHog#61782 no longer reproduces because hideWeekends is ignored and dateRange.daysOfWeek owns weekday filtering.

Changes

• Adds query-runner coverage for every range end, multiple weekends, week starts, timezones, DST, comparisons, breakdowns, and incomplete periods.
• Proves the deprecated hideWeekends field produces the same response when disabled or enabled.
• Preserves a truly empty weekday as zero while populated Monday-through-Thursday buckets retain 3, 3, 3, 12.
• Adds chart-transform coverage and deterministic Storybook stories for all days and weekdays only.
• Leaves production behavior unchanged.

No product UI changes. The Storybook stories provide deterministic visual fixtures for the response shapes.

How did you test this code?

• Ran the targeted Trends pytest matrix to guard response bucket and value alignment.
• Ran the Trends chart-transform Jest suite to guard the response-to-chart boundary.
• Ran frontend formatting, lint fixes, and the full frontend TypeScript check.
• Ran hogli ci:preflight --fix with no failures.
• Rendered both deterministic stories in Storybook with Chromium and verified only Saturday and Sunday disappear.

Automatic notifications

• Publish to changelog?

Docs update

None. This PR adds regression evidence without changing user-facing behavior.

🤖 Agent context

Autonomy: Human-driven (agent-assisted)

Capy traced the historical issue and current query-to-chart path before editing. The investigation found the production fix and later retirement already landed, so this PR adds evidence instead of another behavior patch.

Skills invoked: /writing-ui-components, /writing-tests, /writing-user-facing-copy, /qa-frontend, /setting-feature-flags-in-storybook, /running-ci-preflight, and /writing-pr-descriptions.

All committed fixture data is invented and contains no customer material.

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[github-actions]

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