[bug #67862] rewrite "repmat()" in C++ for performance and classdef

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Follow-up Comment #7, bug #67862 (group octave):

Based on popular demand
https://octave.discourse.group/t/implementation-of-classdef-arrays/7057/18?u=dasergatskov
re-write from a stand-alone function into virtual-method.
(file bug67862_repmat_vmethod_20260402_100.cset)

Perhaps it does make more sense from OO point of view. 
But it does touch 20+ files...
Performance is about the same (it is limited by memory I/O in any case). 

From CSET summary:


  Re-implement repmat as a compiled DEFUN that delegates to a new virtual
  method octave_base_value::repmat().  Each type in the octave_value
  hierarchy overrides the method with an implementation tuned to its
  internal representation. In particular:
    - Dense arrays (octave_base_matrix<MT>): dimension-by-dimension block
      copying with a doubling scheme for contiguous memory access.
    - Scalars (octave_base_scalar<ST>): Array fill constructor.
    - Sparse matrices (octave_base_sparse<T>): direct CSC column construction
      without conversion to dense; errors on non-trivial 3D replication.
    - Structs, old-style @classes, classdef objects: field-by-field or
      object-array replication; classdef checks for an overloaded repmat
      method before falling back to the built-in.
    - Diagonal, permutation, lazy-index, and range types: delegate to
      to_dense() / make_value() and call repmat() on the result.



Dmitri.
-- 


(file #58435)

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