perf(sp1): fast path Reth precompiles

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Summary

Adds SP1-only optimizations to the Reth guest's precompile provider before its existing zkVM syscall fallback:

• specialized MODEXP paths for common bounded operands and exponents
• exact one-entry input/output caches for repeated MODEXP and BLAKE2F calls
• the merged stateless improvements from #30 and #31 via revision 3d2fc17

The Reth validation path still consumes and validates transaction public keys.

Ownership

This belongs in ere-guests because it selects SP1 syscall and cache policy in ERE's Reth crypto adapter. stateless remains zkVM-agnostic; generic MODEXP paths could move into SP1's libzkevm later so the Ethrex guest can use them too.

Correctness

The fast paths are checked against aurora-engine-modexp, including zero/modulus edge cases, U256, fixed-width, Montgomery/REDC, and repeated 248-bit chunk inputs. Both benchmark builds also produced the exact expected output for every fixture.

Full cycle benchmark

Paired SP1 run using zkevm-benchmark-workload at e0b6519, ERE v0.13.0, Reth v2.3.0, SP1 v6.3.1, and four workers. The same 1,077 Osaka 10M cases were converted to v0.13 canonical SSZ through ERE's legacy-to-canonical input bridge.

Build	Total cycles	Exact outputs
Same-base control (cffc3f1 + stateless@3d2fc17)	810,421,889,702	1,077 / 1,077
PR head (5770b2e)	185,706,351,828	1,077 / 1,077
Delta	-624,715,537,874 (-77.085%)	0 mismatches

PR / control = 0.229

PR ELF SHA-256: 08052808b3aeef0f92ac0a83bc9caccf3ed33ef4d21f1043f4b2c3d14370677f

The Zilkworm article reports 159,156,117,864 cycles under the older legacy-input runner. That is useful historical context, but it is not the paired control for the v0.13 result above because the current guest also performs canonical SSZ input/output work.

Checks

• .github/scripts/cargo-fetch-all.sh --locked
• .github/scripts/cargo-fmt-all.sh --check
• cargo clippy --all-targets -- -D warnings
• cargo test --lib --no-fail-fast
• cargo test --doc --no-fail-fast
• SP1 and ZisK feature checks/clippy
• SP1 RISC-V guest build