Iteration states in the forecast guide predate the lifecycle work

[corentin-core]

Context

docs/user/forecast.md describes past iterations of a planned operation as they behaved before the missed-payment lifecycle landed. Three things are now wrong or missing:

• The status table and the state diagram list neither the state where an occurrence has left the forecast nor the one the user chose by declaring the payment never happened.
• "Postponed = late beyond tolerance threshold" conflates the matcher's recognition tolerance with a postponement, which is a decision the user takes.
• "Late iterations remain in the forecast to keep the projection accurate" omits the horizon after which they stop being counted.

Found while shortening the overdue listing window; the file was not touched there because the fix is a rewrite of that section rather than a line edit.

Proposed solution

• Redraw the state diagram from the lifecycle as it is: matched, late, left the forecast, postponed, not counted.
• State the two durations the web app guide already states: counted for a month, listed for one more.
• Replace the tolerance/postponement conflation with the two separate ideas.

Acceptance criteria

• Every state the derivation can produce appears in the guide
• The horizon and the listing window are both stated
• No sentence attributes a postponement to the matcher tolerance

Related issues

• Follows Overdue card lists iterations nobody will ever act on #345