Add tagging system for WorkPrecision and filtering helpers

[ChrisRackauckas-Claude]

Summary

• Adds a tags::Vector{Symbol} field to WorkPrecision for categorizing algorithms (e.g., :rosenbrock, :stiff, :imex, :reference)
• Tags specified via :tags key in setups dicts, propagated through all WorkPrecisionSet constructors (ODE, BVP, RODE, Ensemble, Nonlinear)
• New filtering helpers: filter_by_tags, exclude_by_tags, get_tags, unique_tags, merge_wp_sets
• Plot recipe extended with tags, include_tags, exclude_tags kwargs for direct tag-based filtering in plot calls
• Fully backward compatible — tags default to empty, all existing APIs unchanged

Usage

setups = [
    Dict(:alg => Rosenbrock23(), :tags => [:rosenbrock, :2nd_order, :stiff]),
    Dict(:alg => Rodas5P(),      :tags => [:rosenbrock, :5th_order, :stiff]),
    Dict(:alg => TRBDF2(),       :tags => [:bdf, :2nd_order, :stiff]),
    Dict(:alg => KenCarp4(),     :tags => [:sdirk, :4th_order, :stiff, :imex]),
    Dict(:alg => Tsit5(),        :tags => [:rk, :5th_order, :nonstiff, :reference]),
    Dict(:alg => CVODE_BDF(),    :tags => [:bdf, :sundials, :reference]),
]
wp_set = WorkPrecisionSet(prob, abstols, reltols, setups; appxsol=test_sol)

# Filter and plot
plot(wp_set, tags=(:rosenbrock,))
plot(wp_set, tags=(:5th_order,), include_tags=(:reference,))
rosenbrock_set = filter_by_tags(wp_set, :rosenbrock)
no_reference = exclude_by_tags(wp_set, :reference)

Test plan

• All existing tests pass (benchmark, convergence, stochastic WP, plot recipes, etc.)
• New tagging tests: 24 tests covering all new functionality
• Backward compatibility: setups without :tags key produce empty tags
• Works with all problem types (ODE, BVP, RODE, Ensemble, Nonlinear)

Part of #177 (Phase 1: Core tagging infrastructure)

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