perf(guest): read the private input zero-copy via ef_io::read_input

[diegokingston]

get_private_input() to_vec()'s the whole memory-mapped input before rkyv deserializes it; read_input hands rkyv a slice straight into the input region instead. Same bytes, same private-input commitment.

Measured vs origin/main (same fixtures, deterministic):
transfers_20 8,732,213 -> 8,692,490 (-39,723)
erc20_20 10,328,222 -> 10,278,822 (-49,400)
mixed_20 9,817,444 -> 9,768,492 (-48,952)

Verified: test_prove_ethrex_empty_block (prove+verify) passes.

[diegokingston]

/bench

[github-actions]

Benchmark — ethrex 20 transfers (median of 3)

_{Table parallelism: auto (cores / 3)}

Metric	main	PR	Δ
Peak heap	50217 MB	50237 MB	+20 MB (+0.0%) ⚪
Prove time	24.695s	24.813s	+0.118s (+0.5%) ⚪

✅ No significant change.

✅ Low variance (time: 1.1%, heap: 0.9%)

_{Commit: 75f4083 · Baseline: cached · Runner: self-hosted bench}