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	3.2 ± 0.1	3.1	3.4	1.00

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
head hashmap	138.6 ± 2.1	135.3	141.8	1.00

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
head keccak	131.8 ± 3.7	127.3	138.3	1.00

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
head syscall_commit	91.5 ± 2.4	90.2	98.2	1.00

[github-actions]

Benchmark Results for unmodified programs 🚀

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
base binary_search	59.6 ± 0.7	58.7	60.6	1.00
head binary_search	63.4 ± 1.2	62.0	66.3	1.06 ± 0.02

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
base bitwise_ops	58.5 ± 0.2	58.2	58.9	1.00
head bitwise_ops	62.2 ± 1.1	61.1	64.0	1.06 ± 0.02

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
base fibonacci_26	63.4 ± 0.7	62.7	64.7	1.00
head fibonacci_26	68.0 ± 0.9	66.9	69.6	1.07 ± 0.02

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
base matrix_multiply	65.4 ± 0.6	64.8	67.0	1.00
head matrix_multiply	69.3 ± 1.4	67.2	71.7	1.06 ± 0.02

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
base modular_exp	60.4 ± 1.5	58.6	63.1	1.00
head modular_exp	62.5 ± 0.7	61.4	63.9	1.04 ± 0.03

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
base quicksort	63.5 ± 0.5	62.6	64.0	1.00
head quicksort	67.7 ± 0.5	66.8	68.5	1.07 ± 0.01

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
base sieve	65.4 ± 0.6	64.7	66.2	1.00
head sieve	69.5 ± 0.5	69.1	70.8	1.06 ± 0.01

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
base sum_array	74.1 ± 1.1	73.3	76.8	1.00
head sum_array	78.3 ± 0.6	77.6	79.2	1.06 ± 0.02