CI: add downgrade-compat workflow

[MilesCranmerBot]

Adds a Downgrade Compat GitHub Actions workflow using julia-actions/julia-downgrade-compat@v2 (mode: deps, projects: ., test) to validate compat lower bounds by running tests against a minimized resolver-generated manifest.

For runtime reasons this currently runs the unit/basic test group (via TEST_GROUP=unit/basic).

Fork CI run: https://github.com/MilesCranmerBot/SymbolicRegression.jl/actions/runs/22036091797

[github-actions]

Benchmark Results (Julia v1)

Time benchmarks

	master	929c501...	master / 929c501...
search/multithreading	15.1 ± 0.21 s	15 ± 0.23 s	1 ± 0.02
search/serial	32.5 ± 0.22 s	32.5 ± 0.11 s	0.998 ± 0.0076
utils/best_of_sample	1.88 ± 0.54 μs	1.79 ± 0.49 μs	1.05 ± 0.42
utils/check_constraints_x10	16.7 ± 4.3 μs	16.7 ± 4.2 μs	1 ± 0.36
utils/compute_complexity_x10/Float64	2.15 ± 0.082 μs	2.18 ± 0.091 μs	0.986 ± 0.056
utils/compute_complexity_x10/Int64	2.07 ± 0.091 μs	2.08 ± 0.09 μs	0.995 ± 0.061
utils/compute_complexity_x10/nothing	1.51 ± 0.081 μs	1.51 ± 0.081 μs	1 ± 0.076
utils/insert_random_op_x10	5.3 ± 2 μs	5.11 ± 1.9 μs	1.04 ± 0.55
utils/next_generation_x100	0.435 ± 0.024 ms	0.438 ± 0.021 ms	0.993 ± 0.071
utils/optimize_constants_x10	0.036 ± 0.0079 s	0.0348 ± 0.0081 s	1.03 ± 0.33
utils/randomly_rotate_tree_x10	8.33 ± 0.97 μs	8.26 ± 0.95 μs	1.01 ± 0.17
time_to_load	2.68 ± 0.033 s	2.63 ± 0.022 s	1.02 ± 0.015

Memory benchmarks

	master	929c501...	master / 929c501...
search/multithreading	0.204 G allocs: 53.5 GB	0.206 G allocs: 53 GB	1.01
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	0.041 k allocs: 1.62 kB	0.04 k allocs: 1.56 kB	1.04
utils/next_generation_x100	4.63 k allocs: 0.276 MB	4.62 k allocs: 0.275 MB	1
utils/optimize_constants_x10	21.9 k allocs: 22.2 MB	26.4 k allocs: 28.5 MB	0.777
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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.97%. Comparing base (2f8a7ac) to head (b985468).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #567      +/-   ##
==========================================
- Coverage   95.02%   94.97%   -0.06%     
==========================================
  Files          42       42              
  Lines        3882     3882              
==========================================
- Hits         3689     3687       -2     
- Misses        193      195       +2     

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