Document forecast behavior and add post-integration regression coverage

[kmatsunami]

Summary

Finish the forecast feature by tightening regression coverage around the integrated UI behavior and documenting how forecast differs from pace.

This issue happens after end-to-end integration is in place. It is for docs and post-integration verification only.

Scope

• Update docs to explain the distinction between:
  • pace = comparison against ideal / historical budget
  • forecast = projected end-of-window percentage at the current burn rate
• Document that forecast is only shown when reset time and window duration are known.
• Add or extend tests around integrated row mapping / text formatting that are still missing after #6 and #3.
• Do a final pass for edge cases such as overflow text and no-forecast fallback.

Non-goals

• No core forecast math changes.
• No new UI behavior changes except fixes required to make docs/tests match the intended integrated behavior.

Files likely involved

• docs/ui.md
• Relevant tests under Tests/CodexBarTests

Acceptance Criteria

• Docs clearly distinguish forecast from pace.
• Regression coverage exists for integrated formatting / row mapping that was not already covered in the core math issue.
• Final UI behavior around missing timing info and overflow text is documented and verified.