feat: add use_constants option to disable real constants

[MilesCranmerBot]

Summary

• add use_constants (default true) to backend Options
• when use_constants=false, random leaf generation no longer creates real-valued constants
• disable constant-specific operations when constants are disabled:
  • should_optimize_constants = false
  • probability_negate_constant = 0.0
  • mutation weights mutate_constant and optimize forced to 0.0
• include use_constants in warm-start incompatibility checks
• add unit test coverage for these behaviors

Validation

• TEST_GROUP=unit/basic julia --project=test test/runtests.jl
• Fork CI green: https://github.com/MilesCranmerBot/SymbolicRegression.jl/actions/runs/22205706290

[github-actions]

Benchmark Results (Julia v1)

Time benchmarks

	master	d74f183...	master / d74f183...
search/multithreading	16.4 ± 0.24 s	15.7 ± 0.2 s	1.04 ± 0.02
search/serial	35.7 ± 0.37 s	34 ± 0.35 s	1.05 ± 0.015
utils/best_of_sample	1.82 ± 0.57 μs	1.81 ± 0.51 μs	1.01 ± 0.42
utils/check_constraints_x10	16.9 ± 4.4 μs	16.8 ± 4.2 μs	1 ± 0.36
utils/compute_complexity_x10/Float64	2.15 ± 0.09 μs	2.14 ± 0.09 μs	1 ± 0.06
utils/compute_complexity_x10/Int64	2.05 ± 0.09 μs	2.04 ± 0.09 μs	1 ± 0.062
utils/compute_complexity_x10/nothing	1.5 ± 0.08 μs	1.51 ± 0.081 μs	0.994 ± 0.075
utils/insert_random_op_x10	5.5 ± 1.7 μs	5.18 ± 1.7 μs	1.06 ± 0.47
utils/next_generation_x100	0.439 ± 0.022 ms	0.434 ± 0.022 ms	1.01 ± 0.072
utils/optimize_constants_x10	0.0386 ± 0.008 s	0.0361 ± 0.0078 s	1.07 ± 0.32
utils/randomly_rotate_tree_x10	8.5 ± 1 μs	8.38 ± 0.98 μs	1.01 ± 0.17
time_to_load	2.82 ± 0.025 s	2.75 ± 0.071 s	1.02 ± 0.028

Memory benchmarks

	master	d74f183...	master / d74f183...
search/multithreading	0.205 G allocs: 53.4 GB	0.204 G allocs: 53.5 GB	0.998
search/serial	0.207 G allocs: 53.8 GB	0.207 G allocs: 53.8 GB	1
utils/best_of_sample	0.038 k allocs: 3.25 kB	0.038 k allocs: 3.25 kB	1
utils/check_constraints_x10	0.034 k allocs: 0.875 kB	0.034 k allocs: 0.875 kB	1
utils/compute_complexity_x10/Float64	0 allocs: 0 B	0 allocs: 0 B
utils/compute_complexity_x10/Int64	0 allocs: 0 B	0 allocs: 0 B
utils/compute_complexity_x10/nothing	0 allocs: 0 B	0 allocs: 0 B
utils/insert_random_op_x10	0.046 k allocs: 1.94 kB	0.041 k allocs: 1.62 kB	1.19
utils/next_generation_x100	4.62 k allocs: 0.276 MB	4.63 k allocs: 0.276 MB	1
utils/optimize_constants_x10	25 k allocs: 25.7 MB	24.5 k allocs: 25.4 MB	1.01
utils/randomly_rotate_tree_x10	0.042 k allocs: 1.34 kB	0.042 k allocs: 1.34 kB	1
time_to_load	0.15 k allocs: 11.2 kB	0.145 k allocs: 11 kB	1.02

[codecov]

Codecov Report

❌ Patch coverage is 47.61905% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.77%. Comparing base (9c6460e) to head (d74f183).

Files with missing lines	Patch %	Lines
src/ParametricExpression.jl	8.33%	11 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #576      +/-   ##
==========================================
- Coverage   94.97%   94.77%   -0.21%     
==========================================
  Files          42       42              
  Lines        3883     3904      +21     
==========================================
+ Hits         3688     3700      +12     
- Misses        195      204       +9     

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