Move mat and trunc_series into num/, add num/linear_fn

[devin-ai-integration]

Summary

mat<num, N> and trunc_series<num, N> move verbatim out of fft/engines/algebras.hpp into num/mat.hpp and num/trunc_series.hpp (now in namespace wala instead of wala::fft::engines); the componentwise wrapper engines are unchanged and pick them up by include.

New num/linear_fn.hpp:

// x -> a * x + b; operator* is composition: (f * g)(x) = f(g(x)).
// Default-constructs to the identity.
template <typename num> struct linear_fn { num a = 1; num b = 0; ... };

with operator(), *, *=, ==, plus a small Catch2 test.

The three verify/ds/ affine segtree solutions drop their local linear_fn structs and use wala::linear_fn<num> (f * lazy for composition, default construction for identity, and point_set_range_composite folds the query directly via x = seg[a](x)). All three re-verified AC locally; unit tests pass.

Link to Devin session: https://app.devin.ai/sessions/a0c142d72b3848a8926b2fc111685565
Requested by: @ecnerwala

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✅ Devin Review: No Issues Found

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