Couple Phenology into veg3/JULES-RED

# Couple Phenology into veg3/JULES-RED

## Overview
This issue links the LAI estimated in the RED/veg3 demography scheme to the pre-existing phenology code in JULES, coupling the phenology routines to the veg3/JULES-RED component. Alongside the coupling, the initialisation and diagnostics have been updated to allow a reasonable scientific comparison between the older veg2/TRIFFID vegetation dynamics scheme and RED, and the RED dynamic/veg3 coupling code has been refactored and tidied to ease future development.

## Links
Development branch: [jules_red_veg3_couple_phenology](https://github.com/mo-arthurargles/jules_red_veg3/tree/jules_red_veg3_couple_phenology)

Pull request: [MetOffice/jules#160](https://github.com/MetOffice/jules/pull/160)

Evaluation rose suite for JULES-RED phenology coupling: [u-ec255@366212](https://code.metoffice.gov.uk/trac/roses-u/browser/e/c/2/5/5/trunk?rev=366212), a temperate Fluxnet site in Belgium (BE), run 2006-2015.

Evaluation presentation: [JULES_RED_Phenology_evaluation.pptx](https://github.com/user-attachments/files/31832610/JULES_RED_Phenology_evaluation.pptx)

## Scientific Evaluation
The evaluation suite was designed to answer three questions:

1. Do we get a correct seasonal cycle in terms of leaf turnover and leaf area index (LAI)?
2. Do we get an identical simulated state after doing a dump restart for the final year of simulation?
3. Can we run with independent `phenol_period` (kept at 1 day) and `triffid_period` (set to 10 days)?

### Key findings

- **Seasonal cycle**: JULES-RED reproduces realistic seasonal cycles in leaf turnover and LAI, with phenological behaviour broadly consistent with JULES-TRIFFID for both BDT and NET plant functional types.
- **Litterfall treatment**: The updated litterfall treatment prevents cumulative leaf litter losses from exceeding the available leaf carbon pool, improving physical consistency for deciduous trees (JULES-TRIFFID's litter loss can exceed the leaf carbon pool by 2-4x for BDT).
- **Assimilate**: For BDT, JULES-RED avoids removing more leaf carbon than is actually present, resulting in less of a carbon "tax" during winter compared to TRIFFID.
- **Dump restarts**: Dump restart tests show identical values for assimilate, LAI-related variables (`LAI`, `LAI_BAL`, `LAI_PHEN`), and the prognostic `plantNumDensity` after restart.
- **Independent update periods**: Simulations run correctly with independent `phenol_period` (1 day) and `triffid_period` (10 days) settings, with vegetation dynamics updated on the expected timescale.s.

## Previous links/versions
This issue has been split off from the old MOSRS ticket [#1357](https://code.metoffice.gov.uk/trac/jules/ticket/1357).