Feat/fext accelerator

[jotabulacios]

Motivation

Recursion — the STARK verifier running as an in-VM guest — spends most of its
arithmetic in the VM's native degree-3 Goldilocks extension field
Fp3 = Fp[x]/(x³ − 2) (DEEP composition, FRI folding, constraint evaluation). As
plain RISC-V, one Fp3 multiply-add expands to a dozen-plus field mul/adds plus
the surrounding load/store traffic, all charged to the CPU, MEMW and LT tables.

This PR adds the FEXT precompile (spec PR #809, spec/fext): a·b + c over Fp3
in a single ecall, backed by a small degree-2 AIR — the Fp3 analog of the existing
KECCAK/ECSM accelerators

[github-actions]

Benchmark Results for modified programs 🚀

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
head ecsm	2.6 ± 0.1	2.4	2.9	1.00

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
head hashmap	91.2 ± 3.5	87.3	99.3	1.00

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
head keccak	96.4 ± 4.7	89.1	104.7	1.00

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
head syscall_commit	74.0 ± 2.2	70.7	77.7	1.00

[github-actions]

Benchmark Results for unmodified programs 🚀

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
base binary_search	43.2 ± 1.6	41.2	46.0	1.02 ± 0.05
head binary_search	42.2 ± 1.2	40.9	44.5	1.00

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
base bitwise_ops	44.1 ± 2.3	41.5	47.8	1.00
head bitwise_ops	46.3 ± 2.9	41.9	50.6	1.05 ± 0.09

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
base fibonacci_26	45.1 ± 1.1	43.8	46.7	1.02 ± 0.04
head fibonacci_26	44.1 ± 1.1	42.7	45.6	1.00

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
base matrix_multiply	48.9 ± 1.6	46.8	52.1	1.00
head matrix_multiply	50.1 ± 5.0	43.3	57.2	1.03 ± 0.11

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
base modular_exp	42.2 ± 1.2	41.2	44.9	1.00
head modular_exp	43.2 ± 1.5	41.4	45.5	1.02 ± 0.05

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
base quicksort	44.9 ± 1.6	43.3	48.5	1.00
head quicksort	49.9 ± 2.9	45.5	53.3	1.11 ± 0.08

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
base sieve	47.8 ± 3.1	45.2	52.7	1.03 ± 0.07
head sieve	46.3 ± 1.2	44.7	47.9	1.00

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
base sum_array	53.6 ± 1.2	51.6	55.0	1.00
head sum_array	54.7 ± 1.7	52.5	58.2	1.02 ± 0.04