[AdamJ]

Summary

Task durations and a day's end time could go negative. endDay rounds the end time to the nearest 15 minutes while a running task's start is unrounded, so ending a day shortly after starting a task (task started 9:20, day ended 9:21, end rounded to 9:15) produced a negative duration and an end time before the task began. This PR removes that path and the other places that subtract timestamps without a floor, and rejects end-before-start entries in the archive editors.

Type of Change

• New feature
• Update to existing feature
• Bug fix
• Documentation update
• Other (describe below)

Related Issue

No linked issue.

Changes Made

• src/utils/timeUtil.ts: new zero-floored elapsedMs, plus isEndBeforeStart and END_BEFORE_START_MESSAGE. formatDuration and formatDurationLong show negatives as zero.
• src/contexts/TimeTrackingContext.tsx: endDay never ends earlier than the day start, the running task's start, or the latest recorded task end. beginTask, adjustTaskTime, and the planned-task completion path use elapsedMs. adjustTaskTime collapses an end-before-start edit to a zero-length task.
• src/utils/calculationUtils.ts, src/utils/reportUtils.ts: ignore or zero negative values already in storage (task, day, period, hours, revenue, report formatting).
• src/components/ArchivedTaskRow.tsx, src/components/ArchiveEditDialog.tsx: an end before the start is flagged inline (role="alert", aria-invalid, aria-describedby), and Save/Add Task stays disabled until corrected. The day form rejects it through its zod schema, and Save Changes reopens a collapsed summary editor so the error is visible. Equal start and end times stay allowed, since 15-minute rounding can collapse a short task to zero length.
• src/components/ui/scroll-time-picker.tsx: TimePicker accepts aria-invalid.
• Tests added in TimeTracking.test.tsx, calculationUtils.test.ts, timeUtil.test.ts, ArchivedTaskRow.test.tsx, and ArchiveEditDialog.test.tsx.
• Docs: CHANGELOG.md, README.md, README-EXT.md, AGENTS.md (new "Time Tracking Invariants" section), and agents/pitfalls.md.

Checklist

Documentation

• Any notable changes added to the README.md
• CHANGELOG.md updated accordingly
• N/A — no README changes

General

• Branch is up to date with main
• No unrelated files included in this PR
• Tested locally by invoking the pnpm test, pnpm lint, and pnpm build

Notes for Reviewers

• The new endDay and adjustTaskTime context tests and the four archive validation tests fail without the fix and pass with it.
• Negative values already saved by earlier versions are not rewritten. Calculations ignore them and displays show zero. A legacy negative archived task or day will block saving in the archive editor until its end time is corrected, which is intended.
• TaskEditDialog (current-day task edit) does not show the inline validation. It relies on the adjustTaskTime clamp, so an end-before-start edit there saves as zero length. That could be a follow-up if you want the same inline error.
• docs/TESTING.md, agents/testing.md, and docs/CI_CD.md carry test counts that were already stale. I left them alone.

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