Test Preamble Consolidation

Goal

Replace the near-identical using/import block at the top of each test module with a single shared test/test_helpers/preamble.jl that each module includes.

Design

preamble.jl is included inside each test module and contains:

using Test, RandLinearAlgebra, Random, LinearAlgebra, SparseArrays, StatsBase
import LinearAlgebra: mul!
include("field_test_macros.jl")
include("approx_tol.jl")
using .FieldTest
using .ApproxTol

field_test_macros.jl and approx_tol.jl are no longer included in runtests.jl as this would be redundant.

Rarely used packages (e.g. Hadamard), import Base.*, import RandLinearAlgebra: ..., import Random: seed!, RNG seeds, and test-specific structs remain within each file.

Files

File	Purpose	Planned change
`test/test_helpers/preamble.jl`	Shared module preamble	New
`test/Compressors/**`	Compressor and distribution tests	Replace preamble with `include`, drop redundant imports
`test/Approximators/**`	Approximator and selector tests	Same
`test/Solvers/**`	Solver, error, logger, subsolver tests	Same; two files already self-include helpers
`test/runtests.jl`	Test driver	Remove redundant helper includes (last increment)

No functions are added.

Status

[x] 1. preamble.jl + pilots (gaussian.jl, rangefinder.jl, kaczmarz.jl)
[x] 2. Remaining Compressors files
[x] 3. Remaining Approximators files
[x] 4. Remaining Solvers files and runtests.jl cleanup (full Pkg.test() passes)

Out of scope / potential issues

Per-file RNG seeds could be centralized later.
julia --project=docs/ docs/make.jl currently fails on four unresolved @ref links (RangeFinder in api/approximators.md, Gaussian in manual/compression.md, SparseSign in api/compressors.md, Identity in api/selectors.md). These are in files untouched by this work; with warnonly = [:cross_references] the build succeeds.

Decisions log

Preamble includes FieldTest/ApproxTol itself so its full scope is visible in one file.
Broad using for Test, RandLinearAlgebra, Random, LinearAlgebra, SparseArrays, StatsBase; single-use packages stay local.
Plan change (increment 4): Solvers/SubSolvers/lq.jl and qr.jl were bare @testset blocks, not modules, that relied on runtests.jl loading FieldTest/ApproxTol at the top level (via using ..FieldTest, though neither file used them). They are now wrapped in modules (LQ_subsolver, QR_subsolver) that include the preamble, matching every other test file. This made it safe to remove the helper includes from runtests.jl.