fix(hasher): make the deterministic hasher endian-independent

[vincent-k2026]

Summary

• Fixes bug: bucket_id hash is non-deterministic on big-endian targets #127: the vendored AHash fallback read byte slices via zerocopy::transmute!, i.e. in native endianness, so bucket_id — a consensus-critical hash — would map the same key to different buckets on big-endian targets.
• ReadFromSlice now uses explicit u{16,32,64,128}::from_le_bytes. Little-endian is the canonical interpretation: every current deployment target (x86_64 sequencer, riscv64 fault-proof/zkVM clients, wasm32) is little-endian, so this is bit-identical on all of them.
• The rest of the pipeline was audited and is already endian-independent: fallback.rs is pure integer arithmetic, hash_with_nonce already uses to_le_bytes for the nonce, and write_usize widens by value (also safe on 32-bit wasm).
• The Convert trait with its ~40 transmute! impls was dead vendored code beyond these four slice reads, and this was salt's only use of zerocopy, so both are removed (Cargo.lock updated accordingly).

Why this is consensus-safe to merge

The mutation-testing PR (#140) pinned the exact outputs of hash(), hash_with_nonce(), bucket_id(), and read_small() as unit tests. Those tests passing in CI proves the network format is byte-for-byte unchanged on little-endian; big-endian targets now compute the same canonical values instead of diverging.

🤖 Generated with Claude Code

[github-actions]

Performance Benchmark Comparison

Compared 5 benchmark(s) against the latest main baseline.

Detailed Comparison

Benchmark	Baseline Throughput (Kelem/s)	New Throughput (Kelem/s)	Change
update 10000 KVs/1 threads	75.23	70.14	-6.76%
update 10000 KVs/2 threads	139.09	129.81	-6.67%
update 10000 KVs/4 threads	252.19	238.13	-5.58%
update 10000 KVs/8 threads	437.08	402.89	-7.82%
update 10000 KVs/16 threads	586.88	543.50	-7.39%

[github-actions]

Mutation testing - PASS

Mutation score: 100.0% (36/36 viable mutants killed)

• caught: 36
• survived, real gaps: 0
• timed out, inconclusive: 0
• suppressed: 0
• unviable: 0
• timeout total: 0

No unsuppressed survivors or timeouts remain.

[flyq]

Stale doc line. salt/src/state/ahash/mod.rs:11 still reads "Convert utilities extracted from AHash for efficient byte operations" — the Convert trait is gone. Suggest something like "Little-endian byte-slice readers for the deterministic hasher." (Keeping the convert.rs filename itself is fine — it matches the upstream AHash file and aids diffing against it.)