Improve explicit imports hygiene

[ChrisRackauckas-Claude]

Summary

• Fixed implicit imports in src/DimensionalPlotRecipes.jl by making imports explicit
• Added ExplicitImports.jl tests to ensure import hygiene is maintained
• All tests pass including new ExplicitImports checks

Changes Made

src/DimensionalPlotRecipes.jl

• Changed using RecipesBase → using RecipesBase: RecipesBase, @recipe
• Changed using LinearAlgebra → using LinearAlgebra: LinearAlgebra, norm

This makes the imports explicit and avoids relying on all exports from these packages.

test/explicit_imports.jl (new file)

Added tests to verify:

• No implicit imports in the package
• No stale explicit imports (unused imports)

test/runtests.jl

• Added include("explicit_imports.jl") to run import hygiene tests

Project.toml

• Added ExplicitImports to [extras] and [targets] for test dependencies

Test Results

All tests pass:

Test Summary:   | Pass  Total  Time
ExplicitImports |    2      2  1.4s
     Testing DimensionalPlotRecipes tests passed

@ChrisRackauckas - This PR ensures the package follows best practices for explicit imports and prevents future import hygiene issues through automated testing.

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