refactor: rename seeding/caching terminology to hosting

[netsirius]

Problem

freenet-stdlib still uses outdated "seeding" and "caching" terminology for contracts
a node stores locally. freenet-core already standardized on "hosting" (PR #3522),
but the stdlib types were out of scope and still use the old names, creating an
inconsistency across the ecosystem.

Solution

Rename all seeding/caching types and fields to use "hosting" terminology:

Old	New
SystemMetrics.seeding_contracts	SystemMetrics.hosting_contracts
NodeQuery::ProximityCacheInfo	NodeQuery::NeighborHostingInfo
QueryResponse::ProximityCache(ProximityCacheInfo)	QueryResponse::NeighborHosting(NeighborHostingInfo)
struct ProximityCacheInfo	struct NeighborHostingInfo
struct ContractCacheEntry	struct ContractHostingEntry
struct NeighborCacheInfo	struct NeighborHostingDetail
struct ProximityStats	struct HostingStats

Fields within renamed structs were also updated for consistency:

• cache_hash → hosting_hash
• cached_since → hosted_since
• cache_announces_sent/received → hosting_announces_sent/received
• avg_neighbor_cache_size → avg_neighbor_hosting_size

Breaking change

Version bumped from 0.3.5 → 0.4.0 because all renamed types and fields
are public API. Downstream consumers (freenet-core, river) must update to
the new names.

Serde compatibility note

These types are used for local client → node diagnostics queries only (not
peer-to-peer wire protocol), so the field rename does not affect network
compatibility between nodes.

Testing

• cargo test — 15 passed, 2 ignored (pre-existing)
• cargo check — clean compilation
• Verified no remaining references to old names via grep

Fixes

Closes freenet/freenet-core#3524 (stdlib portion)

[iduartgomez]

before merging this, have a PR ready in freenet core PR for merging it, and test out against this branch all works as intended

[netsirius]

@iduartgomez I have this branch prepared https://github.com/freenet/freenet-core/tree/refactor/rename-seeding-to-hosting