Concatenate scalars with layout arrays via typed_vcat/typed_hcat

[dlfivefifty]

[2; v] with v a layout array (e.g. BandedMatrix(1 => 1:∞)[band(1)] or view(A, 2:∞, 2)) dispatched to Base's generic cat, which never reaches the ArrayLayouts.typed_vcat(T, size, A...) hook. For infinite lazy arrays this hangs, because the result is allocated as a zeros(∞) cache and v is copied into it, whereas [[2]; v] already gives a lazy Vcat.

This PR adds Base.typed_vcat/Base.typed_hcat methods for one or two scalars followed by a LayoutVecOrMats, routing them through the existing hook. The finite fallback uses Base._cat_t when scalars are present (as Base does), so finite results are unchanged.

With InfiniteArrays, the following now all return a lazy Vcat instead of hanging: [2; v], [2; 3; v], vcat(2, v), [2; view(A, band(1))], [2; view(B, 2:∞, 2)], [2; cumsum(1:∞)], [2; (1:∞).^2], [2 view(B, 2:2, 2:∞)].

Merge after JuliaArrays/InfiniteArrays.jl#244: without it, [1; 2; zeros(Int,∞)] would become a Vcat rather than a CachedArray, failing an InfiniteArrays test. With both, the InfiniteArrays and LazyArrays test suites pass.

Not covered: mixed prefixes such as [1.0; 2:3; v] still take the generic path (as [[1]; [2]; v] already does).

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.56%. Comparing base (0d4c00b) to head (f154ca8).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #286      +/-   ##
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+ Coverage   90.33%   90.56%   +0.23%     
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  Files          12       12              
  Lines        1987     2036      +49     
==========================================
+ Hits         1795     1844      +49     
  Misses        192      192              

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