Support adding AbstractPDMats backed by static arrays

[devmotion]

Fixes #167.

+ and pdadd of a PDMat/PDiagMat/ScalMat errored when the underlying storage was a static array, e.g.

PDMat(SMatrix{3,3}(...)) + PDiagMat(SVector{3}(...))
# ERROR: MethodError: no method matching _adddiag(::SMatrix{3,3,Float64,9}, ::SVector{3,Float64})

Root cause

_adddiag/_adddiag! were only defined for Union{Matrix, SparseMatrixCSC} and mutated in place — which is impossible for immutable arrays (and copy(::SMatrix) is still immutable). Subtraction already worked because it routes through Base's broadcasting fallback.

Changes

• Replace the non-mutating _adddiag wrappers with generic AbstractMatrix methods that build the result by construction (a + Diagonal(v), a + v*I, a .+ c.*Diagonal(v)). Allocation-measured to match the previous copy-and-mutate path for dense matrices (1 allocation), stay sparse for sparse inputs, and preserve the static container otherwise.
• Add _scaleadddiag for the matrix-scaled pdadd paths (c*a + diagonal), keeping an in-place fast path for mutable storage (avoids a second allocation) and falling back to muladd for other matrix types.
• Keep the in-place _adddiag! — it is still used by the zero-allocation pdadd! and the dense Matrix(b) fallbacks.
• Vector-taking variants retain @check_argdims to guard against silent broadcast singleton-expansion.

Tests

Added coverage in test/specialarrays.jl for every +/pdadd combination of PDMat{SMatrix} / PDiagMat{SVector} / ScalMat, asserting both numerical correctness and static-type preservation.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.84%. Comparing base (6371e0c) to head (4b9bc14).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #239      +/-   ##
==========================================
+ Coverage   94.78%   94.84%   +0.05%     
==========================================
  Files          11       11              
  Lines         805      814       +9     
==========================================
+ Hits          763      772       +9     
  Misses         42       42              

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[devmotion]

This is required for #238