MoreCast 2.0: PB & Jelly Model Layers

**As a** Weather Forecaster

**I need** to fill empty column cells w/ another chosen model prediction

**So That** I can base my forecast on as much predicted values as possible

**Acceptance Criteria**
- [ ] Given I have a forecast based on a specific model, I can apply any number of other models for the date ranges that are empty
- [ ] Given (Context), When (action carried out), Then (expected outcome)

**Additional Context**
- Ordered Selection of Models: HRDPS, then another, then another, and only when data is indeed available: each successive chosen model will apply its prediction values for the associated dates ONLY if the current value for that date is N/A
- The above is possible for models that only predict short ranges within the whole range
- Removing/Unselecting a model - HOW does this work?

**Definition of Done**
https://github.com/bcgov/wps/wiki/Definition-of-Done

No files, tests, or entry points are named. Start by locating MoreCast's forecast model-selection and date-range handling, then clarify the ordered fallback behavior, unavailable data handling, and how removing a selected model should affect existing values before defining completion tests.

Written by the indexing model from the issue text.