observe: add cost-aware trace sampling

[rickcrawford]

Summary

• Wire proxy.observability.telemetry into runtime startup so the OTLP trace layer is installed with the global subscriber.
• Add source-side parent-based ratio trace sampling with span-end keep overrides for errors, over-budget spans, and slow spans.
• Expose keep_over_budget_usd and keep_slower_than_secs in config parsing, JSON schema, docs, examples, and generated LLM docs.

Validation

• cargo test -p sbproxy-observe telemetry::tests --locked
• cargo test -p sbproxy-observe --locked
• cargo test -p sbproxy-config --locked
• cargo test -p sbproxy --locked
• cargo fmt --all --check
• git diff --check
• ./scripts/check-config-schema.sh
• ./scripts/regen-llms-full.sh --check
• cargo clippy -p sbproxy-observe -p sbproxy-config -p sbproxy --all-targets --locked -- -D warnings

NOTICE

No new third-party libraries were added; NOTICE is unchanged.

Linear: WOR-1214