[Feature]: OpenTelemetry distributed tracing across the boj ↔ ABI ↔ cartridge boundary

[hyperpolymath]

Problem / motivation

Cartridge failures during tools/call are swallowed across MCP client → boj host → Idris2 ABI → cartridge, giving cryptic traces.

Proposed solution

• Propagate W3C traceparent across the Idris2 ABI into the cartridge.
• Structured OpenTelemetry error unwinding so boj returns clear traces to the client.

Rough scope

Medium. Runtime observability.

Part of #274. Re-file of #264 (mis-templated).