Drop the AbstractNamedInteger <: Integer subtyping

[mtfishman]

Summary

Makes AbstractNamedInteger a standalone abstract type instead of <: Integer. A named integer is a tagged scalar, not a true integer: operations like i1 * i2 fuse names rather than multiply cleanly, mixed named and unnamed arithmetic has no well-defined meaning, and inherited Integer fallbacks risk silently dropping the name. The type keeps the explicit integer-like surface it actually needs (equality, hashing, the arithmetic it supports, ordering, show).

This also removes the named-to-plain-number conversion path (the Number constructor and convert) that let named and unnamed integers interoperate. That interoperation is what the standalone design deliberately drops, and a convert that silently discards the name is exactly the name loss being avoided. The one internal consumer, resolving begin and end to a plain index in to_index, now reads the underlying value with denamed, which is also how callers should get the plain integer from a named integer.

The element-type constraints that disambiguate named-range indexing (the BlockArrays getindex methods and the plain-Array indexing method) change from {<:Integer} to {<:AbstractNamedInteger}, since a named unit range's element type is a named integer, which is not an Integer.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.44%. Comparing base (fd324eb) to head (37f4c58).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #172      +/-   ##
==========================================
- Coverage   73.46%   73.44%   -0.03%     
==========================================
  Files          21       21              
  Lines        1142     1141       -1     
==========================================
- Hits          839      838       -1     
  Misses        303      303              

Flag	Coverage Δ
docs	22.78% <0.00%> (+0.02%)	⬆️

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