Summary

Extends the sorted-batch fast path introduced in #526 (AkimaInterpolation) to two more piecewise interpolation types:

• LinearInterpolation
• CubicSpline

When the query vector tt is sorted and the extrapolation modes are simple (None, Constant, Linear, Extension), walks the knots and queries in lockstep in O(n+m) instead of running a per-query binary search via the iguesser. Falls back to map! when the inputs don't fit the fast path: unsorted tt, Periodic/Reflective extrapolation, or (for LinearInterpolation) u containing NaN — preserving the existing NaN-handling semantics from _interpolate(::LinearInterpolation, ...).

The scalar _interpolate, derivative, integral, AD, and plotting paths are unchanged. Output matches the per-point path to floating-point round-off.

Benchmarks

Julia 1.10, n=256 knots, m=4096 sorted queries, BenchmarkTools minimum:

For reference, AkimaInterpolation (post-#526) is at 16.1 μs on the same setup — Linear edges it out slightly with the simpler polynomial; CubicSpline trails due to the extra arithmetic per query.

Unsorted-input fallback stays at ~450 μs for both (issorted short-circuits on the first out-of-order pair, then map! cost dominates).

Why these two

Of the piecewise types, these have the highest per-query baseline cost relative to the binary-search overhead, so they get the largest win from the lockstep walk. Hermite splines (CubicHermiteSpline, QuinticHermiteSpline) and QuadraticSpline are equally amenable to the same pattern — happy to do them as a follow-up if this design is accepted.

What is in this PR

• src/interpolation_methods.jl: adds (A::LinearInterpolation{<:AbstractVector{<:Number}})(out, tt) and (A::CubicSpline{<:AbstractVector{<:Number}})(out, tt) callables, each with their own _*_eval_fast_applicable predicate and _*_eval_sorted! inner loop. The Linear and Extension extrapolation cases collapse to one branch for LinearInterpolation (a line is linear); they're kept separate for CubicSpline since the boundary slope and the continued cubic differ.
• test/interpolation_tests.jl: new "Sorted-batch evaluator" sub-testsets in the Linear and CubicSpline sections, mirroring the coverage from Optimize AkimaInterpolation constructor (3.6-4.1x) and add sorted-batch evaluator (8-9x) #526 (all fast-path extrapolation modes paired left × right, knot pass-through, Periodic/Reflective fallback, None throwing, unsorted fallback, DimensionMismatch, plus cache_parameters=true for CubicSpline and the NaN-in-u fallback for Linear).

Test plan

• CI passes
• Pkg.test() all groups pass locally on Julia 1.10 (Core 2738 / Methods 42151 / Extensions 13178 / Misc 11)
• Output matches per-point path bitwise on the test inputs (verified via max diff print before adding the testset)
• Runic-formatted

Please ignore until reviewed by @ChrisRackauckas.

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