New transformation: SanitiseUnusedRoutineTransformation.

[MichaelSt98]

Introduce a transformation that replaces the executable body of configured routines with a configurable stub while rewriting kept array declarations to fully deferred shape. This allows routines that are no longer called but must still compile to be safely reduced to minimal shells.
The stub_kind parameter controls the replacement body: 'error_stop' (default) inserts a fail-loud ERROR STOP statement so accidental calls are caught at runtime, while 'empty' produces a silent no-op. Routine matching supports both plain names and fully qualified module#routine scheduler item names.
Declarations with intent, pointer, or allocatable attributes are preserved with deferred shape; all other local array declarations are removed. Scalar declarations are kept unchanged.

Description

This pull request introduces the new SanitiseUnusedRoutineTransformation transformation to the codebase, which allows configured routines that are no longer used to be sanitized for compilation. The transformation rewrites array declarations to use deferred shapes, removes unnecessary local arrays, and replaces the routine body with either an error-stop or an empty stub, depending on configuration. Comprehensive tests are added to verify its behavior.

New transformation for sanitizing unused routines:

• Added the SanitiseUnusedRoutineTransformation class in loki/transformations/sanitise/unused_routines.py, which supports rewriting array declarations to deferred shapes, removing unused local arrays, and replacing the body with a configurable stub (error_stop or empty).
• Exposed the new transformation in the loki.transformations.sanitise package by importing it in __init__.py.

Testing and validation:

• Added test_unused_routines.py with comprehensive tests for the new transformation, covering deferred shape rewriting, qualified routine name matching, stub kind selection, non-matching routines, and error handling for invalid stub kinds.

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.

[github-actions]

Documentation for this branch can be viewed at https://sites.ecmwf.int/docs/loki/693/index.html

[codecov]

Codecov Report

❌ Patch coverage is 97.47899% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.21%. Comparing base (bfad794) to head (9b8c12b).
⚠️ Report is 13 commits behind head on main.

Files with missing lines	Patch %	Lines
loki/transformations/sanitise/unused_routines.py	94.11%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #693      +/-   ##
==========================================
+ Coverage   94.19%   94.21%   +0.01%     
==========================================
  Files         288      290       +2     
  Lines       47496    47744     +248     
==========================================
+ Hits        44741    44983     +242     
- Misses       2755     2761       +6     

Flag	Coverage Δ
lint_rules	96.40% <ø> (ø)
loki	94.19% <97.47%> (+0.01%)	⬆️

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