Adopt StructuredError bubbling standard (observability producer)

[AquiGorka]

Adopts the StructuredError error-bubbling standard (already live on provider-platform / pay-platform) on network-dashboard-platform — the producer side of this coordinated pair. Errors are wrapped with context preserving the native cause chain into logs, and a client-safe { code, source, message } shape is what leaves the edge (here, the network WebSocket).

The SDK does not export a StructuredError type, so the shape is defined locally (src/error/structured-error.ts), matching the standard's contract.

What changed

• StructuredError (new, local): carries code + source + native cause; toWire() projects the client-safe shape; idempotent static from() wraps a raw error while preserving it as the cause.
• Logger error() now flattens the .cause chain into outer <- inner <- root, so a wrapped error logs its full context (mirrors provider-platform's flattenCauses).
• council-fetch throws StructuredError — COUNCIL_PLATFORM_HTTP_ERROR on non-OK, COUNCIL_PLATFORM_UNREACHABLE (cause preserved) on network/timeout.
• env config throws StructuredError (ENV_MISSING / ENV_INVALID_NETWORK).
• topology-refresh wraps failures as TOPOLOGY_REFRESH_FAILED (council error as cause → full chain in logs) and broadcasts a client-safe error frame on the bus.
• bus gains a structured-error channel (subscribeErrors / publishError).
• network WS gains an additive { type: "error", error } frame. No subprotocol bump: it's a new frame type, so older SPAs drop it via parseServerFrame rather than mis-render — a platform-first deploy won't disconnect not-yet-updated clients. The 426 body is also structured.

The /public/rpc JSON-RPC error envelope (-32xxx) is left as-is — that's a protocol standard, not this service's own error shape.

Consumer

network-dashboard reads StructuredError.code from the new error frame and maps it to operator copy (its PR consumes this wire contract).

Tests

deno check / lint / fmt / test green. New: StructuredError (wrap/idempotent/toWire), logger cause-chain flattening, topology-refresh error broadcast, WS error-frame delivery + unsubscribe-on-close. 46 tests pass.

Version bump 0.1.13 → 0.1.14 (own commit).