Replace the raw [Option<H256>; 7] sync-roots parameter with a semantic type

Motivation

confirm_replica_sync (and find_matching_root) take a bare [Option<H256>; 7] whose position semantics — index 0 = current snapshot root, indices 1–6 = the prime-bucketed historical-root slots — live only in doc comments and the design doc. Since #274 the array is also a signed payload (the replica attests SCALE(roots)), so its layout is now part of the attestation format, which makes the opacity worse: nothing in the type says what a position means or which side fills it.

Proposal

A named struct in storage-primitives, e.g.

pub struct SyncRoots {
    /// The current snapshot root being attested (position 0 today).
    pub current: Option<H256>,
    /// The six prime-bucketed historical slots (see Bucket.historical_roots).
    pub historical: [Option<H256>; 6],
}

SCALE encodes a struct as the concatenation of its fields, so this is byte-identical to [Option<H256>; 7] — no extrinsic-shape change, no migration, and existing signed attestations stay valid. Constructors like SyncRoots::current(root) replace the node's sync_confirmation_roots helper.

Touch surface

• pallet: confirm_replica_sync parameter, find_matching_root, tests/benchmarks
• provider-node: ReplicaSyncChainClient::submit_sync_confirmation, sync_confirmation_roots, the dynamic-value construction in subxt_client.rs
• crates/storage-subxt regen (type name changes in metadata even though bytes don't)
• JS: descriptors regen; e2e that constructs the array, if any

Related

• #274 — made the array a signed attestation payload
• #301 — touches the same extrinsic's ReplicaSyncRecord bookkeeping; coordinate merge order