Add a standalone observability crate

[lucarlig]

Summary

• add contextforge-data-plane-observability as a standalone workspace crate owning structured JSON formatting, request correlation, W3C trace propagation, OTLP exporters, and performance timers
• emit newline-delimited JSON with stable service, deployment, transaction, correlation, trace, span, pseudonymous user, severity, component, message, and error fields
• preserve or generate request IDs and propagate ContextForge and W3C context to backend transports even when OTLP export is disabled
• record authenticated users only as truncated SHA-256 pseudonyms; never log raw subjects, authorization data, request/response bodies, session IDs, resource URIs, or progress tokens
• emit request, Redis, and backend-call latency as structured PERFORMANCE events
• migrate every production tracing call site to stable messages and queryable fields, and sanitize integration-test/load-driver diagnostics
• document crate ownership, middleware behavior, configuration, filtering fields, and privacy rules

Standards coverage

The implementation covers the applicable logging contract: required common fields, structured error context, transaction/correlation propagation, W3C trace/span IDs, startup metadata, severity conventions, sensitive-data exclusions, and request/database/external-call latency. The dataplane has no application queue or configured CPU/memory alert thresholds, so those conditional event classes are not emitted.

Validation

• cargo fmt --all --check
• cargo clippy --locked --workspace --all-targets -- -D warnings
• cargo nextest run --locked --workspace
• cargo deny check advisories licenses
• cargo build --locked --workspace
• cargo shear --check-test-targets --deny-warnings --locked
• mdbook build _context/wiki
• mdbook test _context/wiki
• startup JSON smoke check
• repository pre-commit hooks, including secret scanning

Not run: the Goose load test because the local dependency stack was not active.