Preserve existing GRIB codes when adding aifs-subs and aifs-subs-crps to BackgroundProcess enum

[Copilot]

Inserting aifs_subs and aifs_subs_crps into the middle of the BackgroundProcess enum shifted all subsequent numeric codes, silently changing the GRIB backgroundProcess values written for existing models and breaking compatibility with existing data and table definitions.

Changes

• Restore original codes: Pre-existing enum values retain their original assignments (aifs_single_mse=3, aifs_ens_crps=4, aifs_ens_diff=5, aifs_compo_single=6, aifs_compo_ens=7, aifs_compo_single_mse=8, aifs_compo_ens_crps=9)
• Allocate new codes for new entries: aifs_subs and aifs_subs_crps are assigned the next available unused codes (10 and 11)

enum class BackgroundProcess : long {
    aifs_single           = 1,
    aifs_ens              = 2,
    aifs_single_mse       = 3,  // restored
    aifs_ens_crps         = 4,  // restored
    aifs_ens_diff         = 5,  // restored
    aifs_compo_single     = 6,  // restored
    aifs_compo_ens        = 7,  // restored
    aifs_compo_single_mse = 8,  // restored
    aifs_compo_ens_crps   = 9,  // restored
    aifs_subs             = 10, // new
    aifs_subs_crps        = 11, // new
    ifs                   = 255
};

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 61.46%. Comparing base (5aacfd4) to head (cf997ef).
⚠️ Report is 1 commits behind head on 178-add-aifs-subs-to-encoder.

Additional details and impacted files

@@                      Coverage Diff                      @@
##           178-add-aifs-subs-to-encoder     #180   +/-   ##
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  Coverage                         61.46%   61.46%           
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  Files                               310      310           
  Lines                             11849    11849           
  Branches                           1040     1040           
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  Hits                               7283     7283           
  Misses                             4566     4566           

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