feat(permutation): batch key derivation API (plan step 1) - #283
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Add PermutationKey::from_seeds, deriving one key per seed with per-seed SeedRejected outcomes, so bulk workloads (ORE batch encryption) get a stable API shape that a future vectorized backend can implement without changing derived output. Pack the shuffle's random words with chunked fill_bytes instead of one next_u64 call per word. A pinned test in vitaminc-random proves the two consume the keystream identically, so seed-derived keys are unchanged. Claude-Session: https://claude.ai/code/session_012cUAnQ2qY9RD5TaAKyZpUm
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Step 1 of the batch/SIMD plan for #280 follow-up. Stacked on #282.
What
PermutationKey::from_seeds: derives one key per seed, in order, each lane exactly equivalent tofrom_seed— independent derivation, independentSeedRejectedoutcome (a rejected seed invalidates only its own lane). Exists for bulk workloads (ORE batch encryption per-block permutations) and fixes the API shape so a future vectorized backend can derive lanes in parallel while staying bit-identical to scalar derivation.random_permutationnow fills its random words via chunkedfill_bytes(64 words per call) instead of onenext_u64call per word — cuts per-call RNG overhead, no output change.Why output is provably unchanged
test_fill_bytes_matches_next_u64_stream(vitaminc-random) pins thatfill_bytesconsumes the ChaCha20 keystream identically to repeatednext_u64calls under little-endian interpretation. If a rand/chacha upgrade ever changes stream consumption, that test fails loudly. The permutation README doctests (seeded, hardcoded outputs) also still pass unchanged.Tests
batch_matches_single_seed_derivation: batch output equals per-seedfrom_seed, order pinned via distinct seedsbatch_of_nothing_is_emptytest_fill_bytes_matches_next_u64_stream: 512-byte fill vs 64next_u64calls, word-by-wordNot in this PR (later steps)
SCHEDULE_256for 8-bit ORE blockshttps://claude.ai/code/session_012cUAnQ2qY9RD5TaAKyZpUm