refactor(api-types): extract shared cloud wire contract from soth-sync

[secxena]

The SDK and the proxy were silently drifting on the cloud wire format: the proxy converted in-process soth_core::TelemetryEvent -> wire TelemetryEvent via a private map_event in soth-sync, while the SDK shipper just serde_json::json!'d the raw in-process struct. That worked at first but blew up on the cloud as a 422 (topic_cluster_id expected string, got integer 0) — and any future field divergence would silently drop SDK telemetry without affecting proxy telemetry.

Extract the cloud API types and the in-process -> wire conversion into a new pure-types crate soth-api-types. Both soth-sync (proxy) and soth-sdk-core (SDK) depend on it, so a contract change in either direction now propagates to both at once.

Also fix the latent SDK shipper bug that produced the 422:

• send X-Soth-Api-Version: 2026-02-01 (matches soth-sync)
• run each event through soth_api_types::convert::map_event
• ship a real TelemetryBatchRequest envelope (batch_id, device_id_hash="sdk:", proxy_signature="sdk-unsigned"), matching the proxy envelope shape

SOTH_SHIPPER_VERBOSE=1 opts into stderr lines with the POST status + event count so SDK customers can verify batching cadence at runtime.

Verified against the production cloud endpoint:
20 anthropic calls -> 1 POST /v1/edge/telemetry/batch -> 200 (20 events)
…from both soth-py and soth-node, with https_proxy unset (SDK is genuinely proxy-independent).

Build/test parity (CI mirror, locally green):

• cargo fmt --all -- --check
• cargo clippy --workspace --lib --bins --tests
• cargo test --workspace --lib --bins
• cargo test -p soth-conformance-tests --test parity (2/2)
• WASM matrix: soth-detect / soth-classify on wasm32-{wasip1,unknown-unknown}
• cargo build -p soth-py / soth-node
• pytest bindings/soth-py/tests (45 passed, 2 skipped)
• npm test in bindings/soth-node (32/32)

Notes:

• observation_records on TelemetryBatchRequest becomes Option<Vec<serde_json::Value>> so soth-api-types stays free of the soth-telemetry (rusqlite/tokio) transitive closure. The proxy serializes its typed records to Value at the call site; JSON wire shape is identical.
• proxy_signature: "sdk-unsigned" is a documented placeholder for the SDK lane — full ed25519 signing of SDK telemetry remains Phase-2 work, called out in shipper.rs.