feat(telemetry): opt-in OpenTelemetry trace export over OTLP

[Shine-neko]

Opt-in OpenTelemetry distributed tracing, exported over OTLP/gRPC.

What

• New [server.telemetry.traces] config block. Off by default: with it disabled no exporter is built and the server behaves exactly as before.
• ring server start emits one span per HTTP request (stable HTTP-server semantic-convention attributes, span named {method} {route}) and one scheduler.cycle span per reconciliation cycle that has work to do — idle scheduler ticks produce no spans.
• Configurable endpoint, service_name and sampler (always_on, always_off, parent_based_always_on, parent_based_always_off, ratio:<0..1>). Standard OTEL_* environment variables override the TOML values.
• An unreachable collector degrades gracefully: the error is logged and the server keeps running without traces.

Traces only; logs and metrics export are intentionally out of scope here and the config block is shaped to accommodate them later.

Tests

• Unit tests: sampler parsing, config resolution (env > TOML > default), telemetry disabled by default.
• E2E (server/t15): server boots and serves requests with tracing enabled against an unreachable collector, and the startup log confirms export was enabled.
• Full suite green (742 unit tests; server E2E passing aside from two failures that predate this branch).

Docs

• documentation/reference/config-toml.md documents [server.telemetry.traces].