refactor(cast): unify downcast felt252 canonicalization branches

[TomerStarkware]

Closes #NA

Follow-up to #1652. Both arms of the felt252 canonicalization in build_downcast reduce to "subtract PRIME when the felt exceeds a threshold" — 0 for a non-positive destination range, HALF_PRIME for one that straddles zero — so this merges them into a single threshold-select.

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[github-actions]

Benchmarking results

Benchmark for program dict_insert

Open benchmarks

Command	Mean [s]	Min [s]	Max [s]	Relative
Cairo-vm (Rust, Cairo 1)	8.321 ± 0.102	8.235	8.598	5.76 ± 0.08
cairo-native (embedded AOT)	1.444 ± 0.007	1.432	1.458	1.00
cairo-native (embedded JIT using LLVM's ORC Engine)	1.463 ± 0.007	1.450	1.472	1.01 ± 0.01

Benchmark for program dict_snapshot

Open benchmarks

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
Cairo-vm (Rust, Cairo 1)	420.4 ± 4.9	414.9	430.6	1.00
cairo-native (embedded AOT)	1289.7 ± 6.3	1278.5	1296.1	3.07 ± 0.04
cairo-native (embedded JIT using LLVM's ORC Engine)	1312.1 ± 3.6	1306.9	1318.4	3.12 ± 0.04

Benchmark for program factorial_2M

Open benchmarks

Command	Mean [s]	Min [s]	Max [s]	Relative
Cairo-vm (Rust, Cairo 1)	3.598 ± 0.074	3.558	3.805	2.68 ± 0.06
cairo-native (embedded AOT)	1.342 ± 0.006	1.328	1.351	1.00
cairo-native (embedded JIT using LLVM's ORC Engine)	1.355 ± 0.005	1.342	1.361	1.01 ± 0.01

Benchmark for program fib_2M

Open benchmarks

Command	Mean [s]	Min [s]	Max [s]	Relative
Cairo-vm (Rust, Cairo 1)	3.516 ± 0.062	3.480	3.688	2.71 ± 0.05
cairo-native (embedded AOT)	1.296 ± 0.008	1.283	1.307	1.00
cairo-native (embedded JIT using LLVM's ORC Engine)	1.317 ± 0.005	1.310	1.325	1.02 ± 0.01

Benchmark for program linear_search

Open benchmarks

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
Cairo-vm (Rust, Cairo 1)	448.3 ± 4.5	441.5	454.5	1.00
cairo-native (embedded AOT)	1306.3 ± 2.7	1302.6	1311.4	2.91 ± 0.03
cairo-native (embedded JIT using LLVM's ORC Engine)	1331.2 ± 5.4	1323.0	1339.7	2.97 ± 0.03

Benchmark for program logistic_map

Open benchmarks

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
Cairo-vm (Rust, Cairo 1)	383.1 ± 3.4	379.1	390.3	1.00
cairo-native (embedded AOT)	1287.4 ± 5.4	1279.8	1296.1	3.36 ± 0.03
cairo-native (embedded JIT using LLVM's ORC Engine)	1321.1 ± 5.4	1313.5	1328.7	3.45 ± 0.03

[github-actions]

Benchmark results Main vs HEAD.

Base

Command	Mean [s]	Min [s]	Max [s]	Relative
base dict_insert.cairo (JIT)	1.923 ± 0.048	1.848	2.009	1.04 ± 0.03
base dict_insert.cairo (AOT)	1.852 ± 0.018	1.828	1.883	1.00

Head

Command	Mean [s]	Min [s]	Max [s]	Relative
head dict_insert.cairo (JIT)	1.906 ± 0.019	1.885	1.949	1.01 ± 0.01
head dict_insert.cairo (AOT)	1.880 ± 0.011	1.862	1.895	1.00

Base

Command	Mean [s]	Min [s]	Max [s]	Relative
base dict_snapshot.cairo (JIT)	1.671 ± 0.021	1.650	1.712	1.00
base dict_snapshot.cairo (AOT)	1.687 ± 0.015	1.657	1.705	1.01 ± 0.02

Head

Command	Mean [s]	Min [s]	Max [s]	Relative
head dict_snapshot.cairo (JIT)	1.692 ± 0.012	1.670	1.711	1.00
head dict_snapshot.cairo (AOT)	1.721 ± 0.024	1.665	1.738	1.02 ± 0.02

Base

Command	Mean [s]	Min [s]	Max [s]	Relative
base factorial_2M.cairo (JIT)	1.780 ± 0.022	1.747	1.813	1.02 ± 0.04
base factorial_2M.cairo (AOT)	1.747 ± 0.061	1.677	1.847	1.00

Head

Command	Mean [s]	Min [s]	Max [s]	Relative
head factorial_2M.cairo (JIT)	1.766 ± 0.014	1.743	1.794	1.00 ± 0.02
head factorial_2M.cairo (AOT)	1.763 ± 0.024	1.732	1.806	1.00

Base

Command	Mean [s]	Min [s]	Max [s]	Relative
base fib_2M.cairo (JIT)	1.695 ± 0.037	1.655	1.772	1.01 ± 0.03
base fib_2M.cairo (AOT)	1.686 ± 0.038	1.642	1.741	1.00

Head

Command	Mean [s]	Min [s]	Max [s]	Relative
head fib_2M.cairo (JIT)	1.741 ± 0.023	1.719	1.801	1.01 ± 0.03
head fib_2M.cairo (AOT)	1.723 ± 0.038	1.677	1.818	1.00

Base

Command	Mean [s]	Min [s]	Max [s]	Relative
base linear_search.cairo (JIT)	1.728 ± 0.020	1.703	1.759	1.01 ± 0.02
base linear_search.cairo (AOT)	1.713 ± 0.031	1.668	1.760	1.00

Head

Command	Mean [s]	Min [s]	Max [s]	Relative
head linear_search.cairo (JIT)	1.786 ± 0.023	1.758	1.828	1.00
head linear_search.cairo (AOT)	1.789 ± 0.057	1.714	1.899	1.00 ± 0.03

Base

Command	Mean [s]	Min [s]	Max [s]	Relative
base logistic_map.cairo (JIT)	1.733 ± 0.031	1.697	1.795	1.04 ± 0.03
base logistic_map.cairo (AOT)	1.663 ± 0.042	1.599	1.747	1.00

Head

Command	Mean [s]	Min [s]	Max [s]	Relative
head logistic_map.cairo (JIT)	1.770 ± 0.030	1.725	1.818	1.02 ± 0.02
head logistic_map.cairo (AOT)	1.736 ± 0.030	1.705	1.802	1.00

[orizi]

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